ClangFormat: Declare QWidget when needed

Amends 120041905.

Change-Id: I88ecd1507cb1817ea315481b9aed430889995c16
Reviewed-by: Artem Sokolovskii <artem.sokolovskii@qt.io>

src/plugins/clangformat/clangformatconfigwidget.h

@@ -3,6 +3,12 @@
 
 #pragma once
 
+#include <QtGlobal>
+
+QT_BEGIN_NAMESPACE
+class QWidget;
+QT_END_NAMESPACE
+
 namespace CppEditor { class CppCodeStyleWidget; }
 namespace TextEditor { class ICodeStylePreferences; }
 namespace ProjectExplorer { class Project; }