i386's genassym.c needs to define some assembly symbols holding the size
of NFS structures to support NFS_ROOT while booting with a nfs_diskless
structure. For this, it needs to include a few NFS headers, which
require definitions from <sys/mount.h> (fhandle_t, vfs_init_t), which
was removed by commit 72ab129799a2 ("x86: remove sys/mount.h from
genassym.c").
Since recently, <sys/mount.h> has been including <sys/vnode.h>, so needs
"vnode_if.h" to have been generated for the compilation of 'genassym.o'
not to fail. Make sure this is the case (for all architectures for
simplicity) by tweaking the rule for 'genassym.o' in
'sys/conf/kern.post.mk', leaving a comment there so that it can be
removed when i386 is dropped (or if the above-mentioned dependency is
broken).
Fixes: 72ab129799a2 ("x86: remove sys/mount.h from genassym.c")
Sponsored by: The FreeBSD Foundation