graphics/wayland: disable posix_fallocate on FreeBSD < 13 after r527045

On FreeBSD < 13 neither memfd_create exists nor posix_fallocate works
with file descriptors returned by shm_open. As SHM_ANON code wasn't
upstreamed and is not used on FreeBSD 13 just disable posix_fallocate
without version checks.

Reported by: Pete Wright, raichoo
Tested by: Pete Wright, raichoo
Approved by: zeising (x11), Greg V
Differential Revision: https://reviews.freebsd.org/D23856