Undo presumably accidental line deletion

Change-Id: I7ddb3ef9b45cfb95c48c2060dc62acd4ed9d64f9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/qml/qml/qqmlextensionplugin.h

@@ -68,6 +68,7 @@ public:
     virtual void registerTypes(const char *uri) = 0;
     virtual void initializeEngine(QQmlEngine *engine, const char *uri);
 
+private:
     Q_DISABLE_COPY(QQmlExtensionPlugin)
 };