mgetgroups: remove dependency on realloc-gnu

mgetgroups: remove dependency on realloc-gnu

The dependency violates the comment in realloc-gnu, which
says that tests can't depend on realloc-gnu; some tests depend
on mgetgroups, so mgetgroups can't depend on realloc-gnu.
Problem reported by Daniel Richard G. in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-10/msg00056.html>.
* lib/mgetgroups.c (mgetgroups): Don't call realloc (NULL, 0).
* modules/mgetgroups (Depends-on): Depend on realloc-posix,
not realloc-gnu.