Windows QPA: Fix unity build

Undef a macro that clashes with variables in other sources.

Pick-to: 6.6 6.5
Task-number: QTBUG-109394
Change-Id: Id62f886ad64908047fc896f7e48da04759308123
Reviewed-by: Wladimir Leuschner <wladimir.leuschner@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
(cherry picked from commit 9362e4f3fa7c42b9d76fd5b4dd504e749b3bcaab)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -538,6 +538,8 @@ QString QWindowsContext::classNamePrefix()
 #  define xstr(s) str(s)
 #  define str(s) #s
         str << xstr(QT_NAMESPACE);
+#  undef str
+#  undef xstr
 #endif
     }
     return result;