viewSelector: Fix another case of mixing GJS and GObject signals

This is the same issue fixed in commit e08f2a4a04 for several other
classes, but this one somehow slipped through ...

js/ui/viewSelector.js

@@ -77,6 +77,7 @@ Signals.addSignalMethods(TouchpadShowOverviewAction.prototype);
 const ShowOverviewAction = new Lang.Class({
     Name: 'ShowOverviewAction',
     Extends: Clutter.GestureAction,
+    Signals: { 'activated': {} },
 
     _init : function() {
         this.parent();
@@ -135,7 +136,6 @@ const ShowOverviewAction = new Lang.Class({
         this.emit('activated', areaDiff);
     }
 });
-Signals.addSignalMethods(ShowOverviewAction.prototype);
 
 const ViewSelector = new Lang.Class({
     Name: 'ViewSelector',