Annotate geom_md with MODULE_VERSION

This was missed in 74d6c131cbe2 where other geom modules were annotated
with MODULE_VERSION. Again, the problem is the same: we can't detect
that geom_md is loaded into the kernel without it.

This was noticed in release builds on the cluster; mdconfig attempts to
load geom_md because it can't detect it in the kernel, but the cluster
config includes md(4) and does not build the kmod. This problem would
have been masked on hosts with the kmod built, as the kmod attempts to
register the g_md module and fails. With this commit, mdconfig would
not even try to load it again.

Reported by: re (cperciva)
MFC after: 3 days