Fix QVariant in a static build.

The patch cases registration of QVariant handler for widgets.

Task-number: QTBUG-28528
Change-Id: I645e8054bb96db0b92edf5df36f206ec1965ad40
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qapplication.cpp

@@ -579,6 +579,7 @@ void QApplicationPrivate::construct()
 void qRegisterGuiStateMachine();
 void qUnregisterGuiStateMachine();
 #endif
+extern void qRegisterWidgetsVariant();
 
 /*!
   \fn void QApplicationPrivate::initialize()
@@ -590,6 +591,9 @@ void QApplicationPrivate::initialize()
     QWidgetPrivate::mapper = new QWidgetMapper;
     QWidgetPrivate::allWidgets = new QWidgetSet;
 
+    // needed for a static build.
+    qRegisterWidgetsVariant();
+
     if (application_type != QApplicationPrivate::Tty)
         (void) QApplication::style();  // trigger creation of application style
 #ifndef QT_NO_STATEMACHINE