Even more Qt6 porting

Task-number: QTCREATORBUG-24098
Change-Id: Ib6ef0e521483153c9716c9a1870072e836d6b026
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/vcsbase/vcsbasediffeditorcontroller.h

@@ -31,6 +31,10 @@
 
 #include <QProcessEnvironment>
 
+QT_BEGIN_NAMESPACE
+class QTextCodec;
+QT_END_NAMESPACE
+
 namespace Core { class IDocument; }
 namespace Utils { class FilePath; }