QmlDesigner: Fix missing include

Change-Id: Ic445036d3ab6f0b673d8541322c0631ced0aeb91
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
(cherry picked from commit 8b32ff93)
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmldesigner/components/itemlibrary/itemlibrarysectionmodel.cpp

@@ -27,6 +27,8 @@
 
 #include "itemlibraryitem.h"
 
+#include <QDebug>
+
 namespace QmlDesigner {
 
 ItemLibrarySectionModel::ItemLibrarySectionModel(QObject *parent) :