cage: return 0 always in the signal handler

According to the Wayland docs:

If the event source is registered for re-check with
wl_event_source_check(): 0 for all done, 1 for needing a re-check. If
not registered, the return value is ignored and should be zero.

See e.g. http://manpages.ubuntu.com/manpages/cosmic/man3/wl_event_source.3.html

Since we don't register any of these for re-checking, we should
return 0.

cage.c

@@ -100,7 +100,7 @@ handle_signal(int signal, void *data)
 		wl_display_terminate(display);
 		return 0;
 	default:
-		return 1;
+		return 0;
 	}
 }