Define _BSD_SOURCE to make initgroups(3) visible

Index: util-src/pposix.c
--- util-src/pposix.c.orig
+++ util-src/pposix.c
@@ -31,6 +31,11 @@
 #define _DARWIN_C_SOURCE
 #endif
 #endif
+#if defined(__OpenBSD__)
+#ifndef _BSD_SOURCE
+#define _BSD_SOURCE
+#endif
+#endif
 
 #if ! defined(__FreeBSD__)
 #ifndef _POSIX_C_SOURCE
