clang-cl: Fix warning about #include resolution

In file included from ..\..\..\mkspecs\win32-clang-msvc\qplatformdefs.h:40:
..\..\..\mkspecs\win32-clang-msvc/../win32-msvc/qplatformdefs.h(51,10):  warning: #include resolved using non-portable Microsoft search rules as: ..\..\corelib\global/qglobal.h [-Wmicrosoft-include]
 #include "qglobal.h"

Task-number: QTBUG-63512
Change-Id: Id60b599126049f2bb07db22e721ff5b761a9b1fd
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>