signals: Remove a false warning about no return value.

This was reverted before. Yet, there is no use keeping that false warning, or wait until gcc is fixed.

modules/control/signals.c

@@ -118,4 +118,5 @@ static void *SigThread (void *data)
     pthread_sigmask (SIG_UNBLOCK, &set, NULL);
     for (;;)
         pause ();
+    return NULL;
 }