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/mcusupport/mcusupportoptions.h

@@ -13,8 +13,6 @@
 #include <QObject>
 #include <QVersionNumber>
 
-QT_FORWARD_DECLARE_CLASS(QWidget)
-
 namespace Utils {
 class FilePath;
 class PathChooser;