PCH build fix when building with Clang 12

Clang 12 has added a template parameter as "CALLBACK",
which conflicts with the Windows "CALLBACK" define.

Change-Id: I773899d103dcb1a852b555a1900b2de4acb6d96f
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/libs/utils/qtcprocess.cpp

@@ -36,6 +36,7 @@
 #include <QStack>
 
 #ifdef Q_OS_WIN
+#define CALLBACK WINAPI
 #include <qt_windows.h>
 #else
 #include <errno.h>