Make QQuickWindow::forcePolish() private.

Adding public API was not the intention when adding
this slot.

Change-Id: If477917c434b32d5e5cd1fa6c640f1a6b167dd24
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/quick/items/qquickwindow.h

@@ -143,7 +143,6 @@ class Q_QUICK_EXPORT QQuickWindow : public QWindow
 
 public Q_SLOTS:
     void update();
-    void forcePolish();
     void releaseResources();
 
 protected:
@@ -173,6 +172,7 @@ public Q_SLOTS:
 private Q_SLOTS:
     void maybeUpdate();
     void cleanupSceneGraph();
+    void forcePolish();
     void setTransientParent_helper(QQuickWindow *window);
 
 private: