ClassView: Remove unneeded includes

Substitute some includes with forward declarations.

Change-Id: I7638a2759b680cfc4e3dfb10631200bb46453998
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/classview/classviewconstants.h

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include <Qt>
+
 namespace ClassView {
 namespace Constants {