Clean headers of the DiffEditor plugin.

Change-Id: I1c49527f010acbece31849127fc4d182678b6f37
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/plugins/diffeditor/diffeditordocument.h

@@ -35,6 +35,8 @@
 
 #include <coreplugin/textdocument.h>
 
+QT_FORWARD_DECLARE_CLASS(QMenu)
+
 namespace DiffEditor {
 
 class DiffEditorController;