add virtual destructor to QWindowSystemInterfacePrivate::WindowSystemEvent

Add a virtual destructor to
QWindowSystemInterfacePrivate::WindowSystemEvent class. Otherwise, in
QWindowSystemInterface::sendWindowSystemEvents() "delete event" can't
destruct the subclass of WindowSystemEvent properly.

Change-Id: Ic708f4ab61de88cd0015aa6e472d356b2de64583
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/gui/kernel/qwindowsysteminterface_qpa_p.h

@@ -73,6 +73,7 @@ public:
     public:
         explicit WindowSystemEvent(EventType t)
             : type(t), synthetic(false) { }
+        virtual ~WindowSystemEvent() { }
         EventType type;
         bool synthetic;
     };