winrt: Wait for main thread to exit before exit.

This will allow application main to go out of scope and free objects
allocated there.

Change-Id: I7b7199ecf67afe578bac043f16b064c9daaae04a
Task-Id: QTBUG-48760
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>

src/winmain/qtmain_winrt.cpp

@@ -169,6 +169,7 @@ public:
         }).Get());
         Q_ASSERT_SUCCEEDED(hr);
 
+        WaitForSingleObjectEx(mainThread, INFINITE, FALSE);
         DWORD exitCode;
         GetExitCodeThread(mainThread, &exitCode);
         return exitCode;