winrt: Fix main thread dispatcher creation

Don't create a dispatcher for all adopted threads, only the main thread.
This was causing the GUI thread to skip platform dispatcher creation.

Change-Id: Id0de976f9def48e8d58efd20815b6fd18faebefa
Reviewed-by: Andrew Knight <andrew.knight@digia.com>

src/corelib/thread/qthread_winrt.cpp

@@ -197,6 +197,7 @@ QThreadData *QThreadData::current(bool createIfNecessary)
 
         if (winmainThread) {
             g->dispatch();
+            threadData->thread->d_func()->createEventDispatcher(threadData);
             winmainThread = false;
         }
     }
@@ -210,7 +211,6 @@ void QAdoptedThread::init()
 
     d->handle = Q_NULLPTR;
     d->id = 0;
-    d->createEventDispatcher(d->data);
 }
 
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