Add missing styleitem reference for tabbar

This fixes some weird painting regressions as well.

Change-Id: I8a381b70119829d46550993f8a43ab04e088eae2
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/styles/Desktop/TabBarStyle.qml

@@ -49,6 +49,8 @@ Item {
     property int tabHSpace: styleitem.pixelMetric("tabhspace");
     property int tabVSpace: styleitem.pixelMetric("tabvspace");
 
+    StyleItem { id: styleitem }
+
     property Component tab: StyleItem {
         id: styleitem
         elementType: "tab"