messageTray: Add missing property getter

gjs has gotten less forgiving about missing getters/setters, and
commit 6aa1b817 missed the missing getter in the base policy class.

Most notifications use a policy subclass that already provides a
getter, but at least Main.notify() and friends don't; unbreak them
by fixing the base class.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1229


(cherry picked from commit 5e254666b0)

js/ui/messageTray.js

@@ -162,6 +162,10 @@ var NotificationPolicy = GObject.registerClass({
         this.run_dispose();
     }
 
+    get enable() {
+        return true;
+    }
+
     get enableSound() {
         return true;
     }