Fix build with -no-opengl on Windows

Change-Id: Ib29012361a2b7f2f0b0da53a7736bb3f3384ae38
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -3015,6 +3015,8 @@ void *QWindowsWindow::surface(void *nativeConfig, int *err)
     }
 
     return m_surface;
+#else
+    return nullptr;
 #endif
 }