Compile with clang

Add back friend declaration removed in e8105e4.
QListViewPrivate uses QAbstractItemView::
DropIndicatorPosition, which is protected.

Change-Id: Ie9fa9422bf4fb7846637dac24a644d4814cfe622
Reviewed-by: Jonas Gastal <jgastal@profusion.mobi>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@nokia.com>

src/widgets/itemviews/qabstractitemview.h

@@ -367,6 +367,7 @@ private:
 
     friend class QTreeViewPrivate; // needed to compile with MSVC
     friend class QListModeViewBase;
+    friend class QListViewPrivate;
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(QAbstractItemView::EditTriggers)