libpwdgrp: make it coexist with <pwd.h>, <grp.h> and <shadow.h>.

allows to get rid of hacks in libbb.h (extra declaration of setgroups -
 now it comes from grp.h as intended).

libpwdgrp/pwd_grp.c

@@ -396,6 +396,7 @@ struct spwd *getspnam(const char *name)
 }
 #endif
 
+#ifdef THIS_ONE_IS_UNUSED
 /* This one doesn't use static buffers */
 int getpw(uid_t uid, char *buf)
 {
@@ -419,6 +420,7 @@ int getpw(uid_t uid, char *buf)
 
 	return -1;
 }
+#endif
 
 /**********************************************************************/