Fix build with -no-feature-treewidget

Make sure the feature is defined before testing it.

Change-Id: I4d4b555c9f2629c8a3ca58cf52efff279f6a78f5
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/widgets/itemviews/qtreewidgetitemiterator_p.h

@@ -53,8 +53,8 @@
 
 #include <QtCore/qstack.h>
 
-#ifndef QT_NO_TREEWIDGET
 #include "qtreewidgetitemiterator.h"
+#if QT_CONFIG(treewidget)
 
 QT_BEGIN_NAMESPACE
 
@@ -103,6 +103,6 @@ private:
 
 QT_END_NAMESPACE
 
-#endif // QT_NO_TREEWIDGET
+#endif // QT_CONFIG(treewidget)
 
 #endif //QTREEWIDGETITEMITERATOR_P_H