QmlJS: Fix compiulation with namespaces

Amends 125d7c0cce.

Change-Id: I75f2c562aca2e3a00bb0284f39c3951664e2c956
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/qmljs/parser/qmljsast_p.h

@@ -43,9 +43,11 @@
 
 #include <QtCore/qversionnumber.h>
 
-QT_QML_BEGIN_NAMESPACE
-
+QT_BEGIN_NAMESPACE
 class QString;
+QT_END_NAMESPACE
+
+QT_QML_BEGIN_NAMESPACE
 
 namespace QmlJS {
 class Parser;