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.