gui: Fix polkit syntax error

Somehow the ECMAscript compatibility was downgraded.

modules/gui.nix

@@ -37,7 +37,7 @@ lib.mkIf config.sbruder.gui.enable {
 
   security.polkit.extraConfig = lib.optionalString config.sbruder.full ''
     // Potential security problem, but required because there is no graphical polkit authentication agent
-    polkit.addRule((action, subject) => {
+    polkit.addRule(function(action, subject) {
       if (action.id == "org.freedesktop.policykit.exec" && action.lookup("program") == "${config.services.input-remapper.package}/bin/input-remapper-control" && subject.isInGroup("users")) {
         return polkit.Result.YES;
       }