wasm: fix single thread apps build QScopedScopeLevelCounter

QScopedScopeLevelCounter was added and it doesn't really need
threads to function, but apps fail to link without it.

Change-Id: I7b355369079bc11b8b8b45b3ee1bf5ba81cd1953
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/thread/qthread.cpp

@@ -146,6 +146,7 @@ void QAdoptedThread::run()
     // this function should never be called
     qFatal("QAdoptedThread::run(): Internal error, this implementation should never be called.");
 }
+#endif
 
 QScopedScopeLevelCounter::QScopedScopeLevelCounter(QThreadData *threadData)
     : threadData(threadData)
@@ -162,6 +163,7 @@ QScopedScopeLevelCounter::~QScopedScopeLevelCounter()
                       << "scope level to" << threadData->scopeLevel;
 }
 
+#if QT_CONFIG(thread)
 /*
   QThreadPrivate
 */