The file is missing from both linux-headers and linux-lts-headers
$ pacman -Fl linux-headers | grep -F mount.h
linux-headers usr/lib/modules/6.0.11-arch1-1/build/include/linux/mount.h
linux-headers usr/lib/modules/6.0.11-arch1-1/build/include/uapi/linux/cifs/cifs_mount.h
linux-headers usr/lib/modules/6.0.11-arch1-1/build/include/uapi/linux/mount.h
linux-headers usr/lib/modules/6.0.11-arch1-1/build/include/uapi/linux/nfs4_mount.h
linux-headers usr/lib/modules/6.0.11-arch1-1/build/include/uapi/linux/nfs_mount.h
$ pacman -Fl linux-lts-headers | grep -F mount.h
linux-lts-headers usr/lib/modules/5.15.81-1-lts/build/include/linux/mount.h
linux-lts-headers usr/lib/modules/5.15.81-1-lts/build/include/uapi/linux/cifs/cifs_mount.h
linux-lts-headers usr/lib/modules/5.15.81-1-lts/build/include/uapi/linux/mount.h
linux-lts-headers usr/lib/modules/5.15.81-1-lts/build/include/uapi/linux/nfs4_mount.h
linux-lts-headers usr/lib/modules/5.15.81-1-lts/build/include/uapi/linux/nfs_mount.h

What do you need the file for?
Edit:
There are also
fs/pnode.h
fs/internal.h
fs/proc/internal.h
fs/proc/fd.h
fs/nfsd/xdr3.h
fs/nfsd/pnfs.h
fs/nfsd/stats.h
fs/nfsd/nfsfh.h
fs/nfsd/nfsd.h
fs/nfsd/netns.h
fs/nfsd/blocklayoutxdr.h
fs/nfsd/xdr4cb.h
fs/nfsd/acl.h
fs/nfsd/xdr.h
fs/nfsd/auth.h
fs/nfsd/trace.h
fs/nfsd/current_stateid.h
fs/nfsd/export.h
fs/nfsd/flexfilelayoutxdr.h
fs/nfsd/cache.h
fs/nfsd/idmap.h
fs/nfsd/xdr4.h
fs/nfsd/filecache.h
fs/nfsd/vfs.h
fs/nfsd/state.h
fs/quota/quota_tree.h
fs/quota/compat.h
fs/quota/quotaio_v2.h
fs/quota/quotaio_v1.h
fs/xfs/xfs_qm.h
fs/xfs/xfs_attr_item.h
fs/xfs/xfs_symlink.h
fs/xfs/xfs_refcount_item.h
fs/xfs/xfs_discard.h
fs/xfs/xfs_ioctl32.h
fs/xfs/xfs_fsops.h
fs/xfs/xfs_message.h
fs/xfs/xfs_mount.h
fs/xfs/xfs_rmap_item.h
fs/xfs/kmem.h
fs/xfs/xfs_log.h
fs/xfs/xfs_iunlink_item.h
fs/xfs/scrub/xfs_scrub.h
fs/xfs/scrub/btree.h
fs/xfs/scrub/bitmap.h
fs/xfs/scrub/trace.h
fs/xfs/scrub/attr.h
fs/xfs/scrub/common.h
fs/xfs/scrub/scrub.h
fs/xfs/scrub/repair.h
fs/xfs/scrub/health.h
fs/xfs/scrub/dabtree.h
fs/xfs/xfs_trace.h
fs/xfs/xfs_log_priv.h
fs/xfs/mrlock.h
fs/xfs/xfs_bmap_item.h
fs/xfs/xfs_aops.h
fs/xfs/xfs_inode_item.h
fs/xfs/xfs_inode.h
fs/xfs/xfs_trans_priv.h
fs/xfs/xfs_itable.h
fs/xfs/xfs_buf.h
fs/xfs/xfs_iomap.h
fs/xfs/xfs_pwork.h
fs/xfs/xfs_dquot.h
fs/xfs/xfs_xattr.h
fs/xfs/xfs_error.h
fs/xfs/xfs_filestream.h
fs/xfs/xfs_super.h
fs/xfs/xfs_sysfs.h
fs/xfs/xfs_export.h
fs/xfs/xfs_iops.h
fs/xfs/xfs_dquot_item.h
fs/xfs/xfs_buf_item.h
fs/xfs/xfs_linux.h
fs/xfs/xfs.h
fs/xfs/libxfs/xfs_ialloc.h
fs/xfs/libxfs/xfs_sb.h
fs/xfs/libxfs/xfs_inode_buf.h
fs/xfs/libxfs/xfs_ialloc_btree.h
fs/xfs/libxfs/xfs_fs.h
fs/xfs/libxfs/xfs_attr_leaf.h
fs/xfs/libxfs/xfs_health.h
fs/xfs/libxfs/xfs_btree.h
fs/xfs/libxfs/xfs_ag.h
fs/xfs/libxfs/xfs_dir2.h
fs/xfs/libxfs/xfs_bmap.h
fs/xfs/libxfs/xfs_trans_space.h
fs/xfs/libxfs/xfs_attr_remote.h
fs/xfs/libxfs/xfs_attr_sf.h
fs/xfs/libxfs/xfs_alloc.h
fs/xfs/libxfs/xfs_shared.h
fs/xfs/libxfs/xfs_refcount_btree.h
fs/xfs/libxfs/xfs_quota_defs.h
fs/xfs/libxfs/xfs_types.h
fs/xfs/libxfs/xfs_rmap_btree.h
fs/xfs/libxfs/xfs_da_format.h
fs/xfs/libxfs/xfs_defer.h
fs/xfs/libxfs/xfs_errortag.h
fs/xfs/libxfs/xfs_dir2_priv.h
fs/xfs/libxfs/xfs_cksum.h
fs/xfs/libxfs/xfs_inode_fork.h
fs/xfs/libxfs/xfs_da_btree.h
fs/xfs/libxfs/xfs_log_format.h
fs/xfs/libxfs/xfs_ag_resv.h
fs/xfs/libxfs/xfs_trans_resv.h
fs/xfs/libxfs/xfs_bmap_btree.h
fs/xfs/libxfs/xfs_alloc_btree.h
fs/xfs/libxfs/xfs_log_recover.h
fs/xfs/libxfs/xfs_bit.h
fs/xfs/libxfs/xfs_btree_staging.h
fs/xfs/libxfs/xfs_format.h
fs/xfs/libxfs/xfs_refcount.h
fs/xfs/libxfs/xfs_rmap.h
fs/xfs/libxfs/xfs_attr.h
fs/xfs/xfs_pnfs.h
fs/xfs/xfs_bmap_util.h
fs/xfs/xfs_reflink.h
fs/xfs/xfs_extfree_item.h
fs/xfs/xfs_sysctl.h
fs/xfs/xfs_trans.h
fs/xfs/xfs_icache.h
fs/xfs/xfs_ondisk.h
fs/xfs/xfs_mru_cache.h
fs/xfs/xfs_quota.h
fs/xfs/xfs_fsmap.h
fs/xfs/xfs_icreate_item.h
fs/xfs/xfs_iwalk.h
fs/xfs/xfs_extent_busy.h
fs/xfs/xfs_rtalloc.h
fs/xfs/xfs_stats.h
fs/xfs/xfs_ioctl.h
fs/xfs/xfs_acl.h
fs/kernfs/kernfs-internal.h
fs/affs/amigaffs.h
fs/affs/affs.h
fs/hfs/hfs_fs.h
fs/hfs/btree.h
fs/hfs/hfs.h
fs/ksmbd/smbacl.h
fs/ksmbd/ntlmssp.h
fs/ksmbd/transport_ipc.h
fs/ksmbd/nterr.h
fs/ksmbd/crypto_ctx.h
fs/ksmbd/asn1.h
fs/ksmbd/smb_common.h
fs/ksmbd/smb2pdu.h
fs/ksmbd/transport_tcp.h
fs/ksmbd/connection.h
fs/ksmbd/misc.h
fs/ksmbd/glob.h
fs/ksmbd/smbstatus.h
fs/ksmbd/auth.h
fs/ksmbd/ndr.h
fs/ksmbd/smbfsctl.h
fs/ksmbd/ksmbd_netlink.h
fs/ksmbd/oplock.h
fs/ksmbd/mgmt/tree_connect.h
fs/ksmbd/mgmt/share_config.h
fs/ksmbd/mgmt/user_config.h
fs/ksmbd/mgmt/ksmbd_ida.h
fs/ksmbd/mgmt/user_session.h
fs/ksmbd/ksmbd_work.h
fs/ksmbd/xattr.h
fs/ksmbd/unicode.h
fs/ksmbd/vfs_cache.h
fs/ksmbd/vfs.h
fs/ksmbd/uniupr.h
fs/ksmbd/transport_rdma.h
fs/ksmbd/server.h
fs/qnx6/qnx6.h
fs/nfs/filelayout/filelayout.h
fs/nfs/pnfs.h
fs/nfs/delegation.h
fs/nfs/callback.h
fs/nfs/fscache.h
fs/nfs/internal.h
fs/nfs/netns.h
fs/nfs/nfs42.h
fs/nfs/nfs.h
fs/nfs/nfs3_fs.h
fs/nfs/nfs4_fs.h
fs/nfs/dns_resolve.h
fs/nfs/nfs4session.h
fs/nfs/nfs4trace.h
fs/nfs/sysfs.h
fs/nfs/cache_lib.h
fs/nfs/flexfilelayout/flexfilelayout.h
fs/nfs/blocklayout/blocklayout.h
fs/nfs/nfs4idmap.h
fs/nfs/nfstrace.h
fs/nfs/iostat.h
fs/ubifs/ubifs-media.h
fs/ubifs/misc.h
fs/ubifs/key.h
fs/ubifs/debug.h
fs/ubifs/ubifs.h
fs/bfs/bfs.h
fs/netfs/internal.h
fs/adfs/adfs.h
fs/adfs/dir_fplus.h
fs/adfs/dir_f.h
fs/configfs/configfs_internal.h
fs/efivarfs/internal.h
fs/fscache/internal.h
fs/vboxsf/vfsmod.h
fs/vboxsf/shfl_hostintf.h
fs/reiserfs/acl.h
fs/reiserfs/reiserfs.h
fs/reiserfs/xattr.h
fs/sysv/sysv.h
fs/ocfs2/uptodate.h
fs/ocfs2/dlmglue.h
fs/ocfs2/blockcheck.h
fs/ocfs2/symlink.h
fs/ocfs2/dcache.h
fs/ocfs2/journal.h
fs/ocfs2/stackglue.h
fs/ocfs2/ocfs1_fs_compat.h
fs/ocfs2/localalloc.h
fs/ocfs2/dlm/dlmapi.h
fs/ocfs2/dlm/dlmcommon.h
fs/ocfs2/dlm/dlmdebug.h
fs/ocfs2/dlm/dlmconvert.h
fs/ocfs2/dlm/dlmdomain.h
fs/ocfs2/heartbeat.h
fs/ocfs2/super.h
fs/ocfs2/ocfs2_lockid.h
fs/ocfs2/ocfs2_lockingver.h
fs/ocfs2/ocfs2_trace.h
fs/ocfs2/acl.h
fs/ocfs2/filecheck.h
fs/ocfs2/slot_map.h
fs/ocfs2/ocfs2_ioctl.h
fs/ocfs2/suballoc.h
fs/ocfs2/aops.h
fs/ocfs2/mmap.h
fs/ocfs2/locks.h
fs/ocfs2/extent_map.h
fs/ocfs2/sysfile.h
fs/ocfs2/alloc.h
fs/ocfs2/cluster/quorum.h
fs/ocfs2/cluster/tcp.h
fs/ocfs2/cluster/sys.h
fs/ocfs2/cluster/heartbeat.h
fs/ocfs2/cluster/masklog.h
fs/ocfs2/cluster/tcp_internal.h
fs/ocfs2/cluster/nodemanager.h
fs/ocfs2/cluster/ocfs2_nodemanager.h
fs/ocfs2/cluster/ocfs2_heartbeat.h
fs/ocfs2/dlmfs/userdlm.h
fs/ocfs2/ocfs2.h
fs/ocfs2/export.h
fs/ocfs2/buffer_head_io.h
fs/ocfs2/namei.h
fs/ocfs2/ocfs2_fs.h
fs/ocfs2/dir.h
fs/ocfs2/xattr.h
fs/ocfs2/file.h
fs/ocfs2/move_extents.h
fs/ocfs2/resize.h
fs/ocfs2/ioctl.h
fs/ocfs2/reservations.h
fs/ocfs2/refcounttree.h
fs/ocfs2/quota.h
fs/ocfs2/inode.h
fs/smbfs_common/arc4.h
fs/smbfs_common/md4.h
fs/smbfs_common/smb2pdu.h
fs/smbfs_common/smbfsctl.h
fs/omfs/omfs.h
fs/omfs/omfs_fs.h
fs/jffs2/compr.h
fs/jffs2/nodelist.h
fs/jffs2/acl.h
fs/jffs2/os-linux.h
fs/jffs2/summary.h
fs/jffs2/jffs2_fs_i.h
fs/jffs2/jffs2_fs_sb.h
fs/jffs2/debug.h
fs/jffs2/xattr.h
fs/exfat/exfat_raw.h
fs/exfat/exfat_fs.h
fs/dlm/lvb_table.h
fs/dlm/midcomms.h
fs/dlm/lockspace.h
fs/dlm/lowcomms.h
fs/dlm/requestqueue.h
fs/dlm/recover.h
fs/dlm/config.h
fs/dlm/member.h
fs/dlm/rcom.h
fs/dlm/util.h
fs/dlm/ast.h
fs/dlm/recoverd.h
fs/dlm/dir.h
fs/dlm/user.h
fs/dlm/lock.h
fs/dlm/memory.h
fs/dlm/dlm_internal.h
fs/btrfs/free-space-tree.h
fs/btrfs/tree-mod-log.h
fs/btrfs/print-tree.h
fs/btrfs/ctree.h
fs/btrfs/delalloc-space.h
fs/btrfs/locking.h
fs/btrfs/disk-io.h
fs/btrfs/space-info.h
fs/btrfs/qgroup.h
fs/btrfs/inode-item.h
fs/btrfs/async-thread.h
fs/btrfs/subpage.h
fs/btrfs/discard.h
fs/btrfs/zoned.h
fs/btrfs/misc.h
fs/btrfs/tests/btrfs-tests.h
fs/btrfs/props.h
fs/btrfs/ulist.h
fs/btrfs/compression.h
fs/btrfs/extent_io.h
fs/btrfs/delayed-ref.h
fs/btrfs/ordered-data.h
fs/btrfs/volumes.h
fs/btrfs/extent-io-tree.h
fs/btrfs/tree-log.h
fs/btrfs/reflink.h
fs/btrfs/tree-checker.h
fs/btrfs/extent_map.h
fs/btrfs/raid56.h
fs/btrfs/send.h
fs/btrfs/btrfs_inode.h
fs/btrfs/rcu-string.h
fs/btrfs/export.h
fs/btrfs/check-integrity.h
fs/btrfs/sysfs.h
fs/btrfs/ref-verify.h
fs/btrfs/xattr.h
fs/btrfs/backref.h
fs/btrfs/delayed-inode.h
fs/btrfs/dev-replace.h
fs/btrfs/block-group.h
fs/btrfs/free-space-cache.h
fs/btrfs/transaction.h
fs/btrfs/block-rsv.h
fs/cachefiles/internal.h
fs/autofs/autofs_i.h
fs/orangefs/upcall.h
fs/orangefs/orangefs-dev-proto.h
fs/orangefs/downcall.h
fs/orangefs/orangefs-bufmap.h
fs/orangefs/orangefs-debugfs.h
fs/orangefs/orangefs-kernel.h
fs/orangefs/orangefs-debug.h
fs/orangefs/protocol.h
fs/orangefs/orangefs-sysfs.h
fs/ext2/acl.h
fs/ext2/ext2.h
fs/ext2/xattr.h
fs/ramfs/internal.h
fs/unicode/utf8n.h
fs/ecryptfs/ecryptfs_kernel.h
fs/gfs2/incore.h
fs/gfs2/trace_gfs2.h
fs/gfs2/lops.h
fs/gfs2/recovery.h
fs/gfs2/rgrp.h
fs/gfs2/sys.h
fs/gfs2/glops.h
fs/gfs2/super.h
fs/gfs2/acl.h
fs/gfs2/bmap.h
fs/gfs2/aops.h
fs/gfs2/gfs2.h
fs/gfs2/util.h
fs/gfs2/log.h
fs/gfs2/dir.h
fs/gfs2/xattr.h
fs/gfs2/meta_io.h
fs/gfs2/quota.h
fs/gfs2/glock.h
fs/gfs2/trans.h
fs/gfs2/inode.h
fs/hpfs/hpfs_fn.h
fs/hpfs/hpfs.h
fs/hostfs/hostfs.h
fs/erofs/internal.h
fs/erofs/compress.h
fs/erofs/tagptr.h
fs/erofs/zdata.h
fs/erofs/xattr.h
fs/erofs/erofs_fs.h
fs/fuse/fuse_i.h
fs/freevxfs/vxfs.h
fs/freevxfs/vxfs_dir.h
fs/freevxfs/vxfs_inode.h
fs/freevxfs/vxfs_fshead.h
fs/freevxfs/vxfs_olt.h
fs/freevxfs/vxfs_extern.h
fs/minix/minix.h
fs/sysfs/sysfs.h
fs/nilfs2/mdt.h
fs/nilfs2/page.h
fs/nilfs2/btree.h
fs/nilfs2/nilfs.h
fs/nilfs2/cpfile.h
fs/nilfs2/segment.h
fs/nilfs2/sufile.h
fs/nilfs2/bmap.h
fs/nilfs2/direct.h
fs/nilfs2/alloc.h
fs/nilfs2/ifile.h
fs/nilfs2/export.h
fs/nilfs2/btnode.h
fs/nilfs2/sysfs.h
fs/nilfs2/the_nilfs.h
fs/nilfs2/dat.h
fs/nilfs2/segbuf.h
fs/cramfs/internal.h
fs/ufs/ufs.h
fs/ufs/util.h
fs/ufs/ufs_fs.h
fs/ufs/swab.h
fs/overlayfs/ovl_entry.h
fs/overlayfs/overlayfs.h
fs/udf/ecma_167.h
fs/udf/udfdecl.h
fs/udf/udf_sb.h
fs/udf/udf_i.h
fs/udf/osta_udf.h
fs/udf/udfend.h
fs/efs/efs.h
fs/jfs/jfs_superblock.h
fs/jfs/jfs_imap.h
fs/jfs/jfs_unicode.h
fs/jfs/jfs_inode.h
fs/jfs/jfs_types.h
fs/jfs/jfs_xtree.h
fs/jfs/jfs_lock.h
fs/jfs/jfs_logmgr.h
fs/jfs/jfs_xattr.h
fs/jfs/jfs_metapage.h
fs/jfs/jfs_incore.h
fs/jfs/jfs_dinode.h
fs/jfs/jfs_extent.h
fs/jfs/jfs_filsys.h
fs/jfs/jfs_btree.h
fs/jfs/jfs_dmap.h
fs/jfs/jfs_acl.h
fs/jfs/jfs_txnmgr.h
fs/jfs/jfs_dtree.h
fs/jfs/jfs_discard.h
fs/jfs/jfs_debug.h
fs/zonefs/trace.h
fs/zonefs/zonefs.h
fs/debugfs/internal.h
fs/ext4/fsmap.h
fs/ext4/acl.h
fs/ext4/ext4_extents.h
fs/ext4/fast_commit.h
fs/ext4/truncate.h
fs/ext4/mballoc.h
fs/ext4/extents_status.h
fs/ext4/ext4_jbd2.h
fs/ext4/xattr.h
fs/ext4/ext4.h
fs/ntfs/endian.h
fs/ntfs/sysctl.h
fs/ntfs/logfile.h
fs/ntfs/lcnalloc.h
fs/ntfs/types.h
fs/ntfs/mft.h
fs/ntfs/time.h
fs/ntfs/bitmap.h
fs/ntfs/collate.h
fs/ntfs/attrib.h
fs/ntfs/aops.h
fs/ntfs/index.h
fs/ntfs/ntfs.h
fs/ntfs/runlist.h
fs/ntfs/layout.h
fs/ntfs/dir.h
fs/ntfs/debug.h
fs/ntfs/malloc.h
fs/ntfs/quota.h
fs/ntfs/volume.h
fs/ntfs/inode.h
fs/ntfs/usnjrnl.h
fs/crypto/fscrypt_private.h
fs/ntfs3/lib/decompress_common.h
fs/ntfs3/lib/lib.h
fs/ntfs3/ntfs.h
fs/ntfs3/ntfs_fs.h
fs/ntfs3/debug.h
fs/coda/coda_int.h
fs/coda/coda_linux.h
fs/coda/coda_psdev.h
fs/coda/coda_cache.h
fs/coda/coda_fs_i.h
fs/romfs/internal.h
fs/fat/fat.h
fs/f2fs/segment.h
fs/f2fs/acl.h
fs/f2fs/gc.h
fs/f2fs/xattr.h
fs/f2fs/f2fs.h
fs/f2fs/node.h
fs/f2fs/iostat.h
fs/cifs/cifs_unicode.h
fs/cifs/fs_context.h
fs/cifs/ntlmssp.h
fs/cifs/cifspdu.h
fs/cifs/cifsglob.h
fs/cifs/smb2glob.h
fs/cifs/dfs_cache.h
fs/cifs/cifs_debug.h
fs/cifs/nterr.h
fs/cifs/cifsacl.h
fs/cifs/fscache.h
fs/cifs/smb2pdu.h
fs/cifs/cifs_ioctl.h
fs/cifs/cifsfs.h
fs/cifs/cifs_swn.h
fs/cifs/smb2proto.h
fs/cifs/trace.h
fs/cifs/dns_resolve.h
fs/cifs/cifs_spnego.h
fs/cifs/cifsproto.h
fs/cifs/netlink.h
fs/cifs/smbdirect.h
fs/cifs/cached_dir.h
fs/cifs/cifs_uniupr.h
fs/cifs/smb2status.h
fs/cifs/cifs_fs_sb.h
fs/cifs/rfc1002pdu.h
fs/cifs/smberr.h
fs/afs/afs.h
fs/afs/xdr_fs.h
fs/afs/afs_fs.h
fs/afs/internal.h
fs/afs/afs_vl.h
fs/afs/protocol_uae.h
fs/afs/afs_cm.h
fs/afs/protocol_yfs.h
fs/afs/protocol_afs.h
fs/lockd/svcxdr.h
fs/lockd/netns.h
fs/lockd/procfs.h
fs/squashfs/squashfs.h
fs/squashfs/squashfs_fs_sb.h
fs/squashfs/decompressor.h
fs/squashfs/page_actor.h
fs/squashfs/xattr.h
fs/squashfs/squashfs_fs_i.h
fs/squashfs/squashfs_fs.h
fs/qnx4/qnx4.h
fs/befs/endian.h
fs/befs/btree.h
fs/befs/befs.h
fs/befs/super.h
fs/befs/datastream.h
fs/befs/io.h
fs/befs/inode.h
fs/befs/befs_fs_types.h
fs/verity/fsverity_private.h
fs/9p/fid.h
fs/9p/acl.h
fs/9p/v9fs_vfs.h
fs/9p/v9fs.h
fs/9p/cache.h
fs/9p/xattr.h
fs/isofs/rock.h
fs/isofs/isofs.h
fs/isofs/zisofs.h
fs/pstore/internal.h
fs/hfsplus/hfsplus_fs.h
fs/hfsplus/hfsplus_raw.h
fs/hfsplus/xattr.h
fs/iomap/trace.h
fs/notify/inotify/inotify.h
fs/notify/fanotify/fanotify.h
fs/notify/fdinfo.h
fs/notify/fsnotify.h
fs/ceph/super.h
fs/ceph/mds_client.h
fs/ceph/metric.h
fs/ceph/io.h
fs/ceph/cache.h
fs/ceph/ioctl.h

I am building BPF programs for which I need to include mount.h in order to get access to mount and mnt_namespace structures definitions.
This is a small header file, containing important structures definition so I think it is an issue not to have it in the kernel headers package.

It's missing for good reason, it's an internal header. Whatever it is you're trying to do, you'll have to do it differently. Please see here [1] for example.

[1] https://stackoverflow.com/questions/61848764/incomplete-definition-and-forward-declaration-of-type-struct-mnt-namespace-usi