QmlJSEditor: add missing class forward

Change-Id: Ic2f7c8e9366f86f6f768cc21a060112f25c31278
Reviewed-by: Marco Benelli <marco.benelli@qt.io>

src/plugins/qmljseditor/qmljsoutline.h

@@ -31,6 +31,10 @@
 
 #include <QSortFilterProxyModel>
 
+QT_BEGIN_NAMESPACE
+class QAction;
+QT_END_NAMESPACE
+
 namespace Core { class IEditor; }
 
 namespace QmlJS { class Editor; }