Fix a regression in scrollbar

This was a regression after making scrollbar private.

Change-Id: I28858ccebacf2f2572a61af7c558e7e257a45061
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/qtdesktop/ScrollArea.qml

@@ -109,7 +109,7 @@ FocusScope {
 
     onContentXChanged: {
         scroller.blockUpdates = true
-        horizontalScrollBar.value = contentX
+        scroller.horizontalScrollBar.value = contentX
         scroller.horizontalValue = contentX
         scroller.blockUpdates = false
     }