QmlJS: Synchronize Engine::registerValue.

When multiple threads share the Bind instance in a Document and do
lookup on the objects contained inside, that may trigger a new object
creation in some situations - which needs to be synchronized.

src/libs/qmljs/qmljsinterpreter.h

@@ -40,6 +40,7 @@
 #include <QtCore/QString>
 #include <QtCore/QHash>
 #include <QtCore/QSet>
+#include <QtCore/QMutex>
 
 namespace QmlJS {
 
@@ -735,6 +736,8 @@ private:
     TypeId _typeId;
 
     CppQmlTypes _cppQmlTypes;
+
+    QMutex _mutex;
 };