process.c: initgroups on cygwin

* process.c (initgroups): not declared on cygwin if _POSIX_SOURCE
  or _XOPEN_SOURCE are defined.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50304 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

process.c

@@ -85,6 +85,9 @@
 #endif
 #ifdef HAVE_GRP_H
 #include <grp.h>
+# ifdef __CYGWIN__
+int initgroups(const char *, rb_gid_t);
+# endif
 #endif
 #ifdef HAVE_SYS_ID_H
 #include <sys/id.h>