Bug 1848764 [Wayland] Don't use UserIdleServiceX11() on Wayland only builds r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D186209

widget/gtk/nsUserIdleServiceGTK.cpp

@@ -245,9 +245,11 @@ void nsUserIdleServiceGTK::ProbeService() {
       idleService = new UserIdleServiceMutter(this);
       break;
 #endif
+#ifdef MOZ_X11
     case IDLE_SERVICE_XSCREENSAVER:
       idleService = new UserIdleServiceX11(this);
       break;
+#endif
     default:
       return;
   }