Fix deadlocks in debugger infrastructure on Mac

Set QReadWriteLocker to recursive. This is needed e.g. to allow
_q_changeServiceState to get a read lock when called from inside
receiveMessage.

Change-Id: I287a7c7f44e94005c0458825f8f6d1877ee914dd
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>

src/qml/debugger/qqmldebugserver.cpp

@@ -141,6 +141,7 @@ private:
 
 QQmlDebugServerPrivate::QQmlDebugServerPrivate() :
     connection(0),
+    pluginsLock(QReadWriteLock::Recursive),
     gotHello(false),
     blockingMode(false),
     thread(0)