Fix MSVC warnings about struct/class mismatch for dynamic metaobjects.

Change-Id: I263f25161fa1861ca5e7da9440214fd3ba6e944e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/corelib/kernel/qobject.h

@@ -79,7 +79,7 @@ class QRegularExpression;
 #ifndef QT_NO_USERDATA
 class QObjectUserData;
 #endif
-class QDynamicMetaObjectData;
+struct QDynamicMetaObjectData;
 
 typedef QList<QObject*> QObjectList;