Fix signature of other notification backends

Why isn't this a compiler error in the first place..

src/fe-gtk/notifications/notification-osx.m

@@ -37,7 +37,7 @@ notification_backend_show (const char *title, const char *text)
 }
 
 int
-notification_backend_init (void)
+notification_backend_init (const char **error)
 {
 	return 1;
 }