bug#20923: mgetgroups.c vs getgrouplist warning on OS X

[Jim Meyering]

I compiled the just-published snapshot on OS X configured with
--enable-gcc-warnings, and saw this:

lib/mgetgroups.c: In function 'mgetgroups':
lib/mgetgroups.c:90:45: error: pointer targets in passing argument 3
of 'getgrouplist' differ in signedness [-Werror=pointer-sign]
           ng = getgrouplist (username, gid, g, &max_n_groups);
                                             ^