fix memory leak in QGuiApplicationPrivate

delete app_font object in QGuiApplicationPrivate's destructor

Change-Id: Iad0c5a69e03477f6c93fcf76c58bac001cbdffb8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>

src/gui/kernel/qguiapplication.cpp

@@ -484,6 +484,8 @@ QGuiApplicationPrivate::~QGuiApplicationPrivate()
         delete generic_plugin_list.at(i);
     generic_plugin_list.clear();
 
+    delete app_font;
+    app_font = 0;
     QFont::cleanup();
 
 #ifndef QT_NO_CURSOR