Revert "signals: Remove a false warning about no return value."

This reverts commit 8510313d.

Don't commit late things, without reading the justification.

This is generating dead code, and produce a warning on newer gcc version for that.

modules/control/signals.c

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