VariousPlugins: Fix forward declarations

The pattern is to use QT_[BEGIN/END]_NAMESPACE, instead of
QT_FORWARD_DECLARE_CLASS.

Change-Id: I9da39ee003e1d477a5f56980c218e275706bb3f4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/diffeditor/diffview.h

@@ -11,7 +11,9 @@
 #include <QString>
 #include <QObject>
 
-QT_FORWARD_DECLARE_CLASS(QWidget)
+QT_BEGIN_NAMESPACE
+class QWidget;
+QT_END_NAMESPACE
 
 namespace TextEditor { class TextEditorWidget; }