Added missing namespace in Windows

src/GraphicsControls.Sample/GraphicsControls.Sample.WinUI/Platforms/Windows/App.xaml.cs

@@ -1,6 +1,6 @@
 using Microsoft.Maui;
+using Microsoft.Maui.Hosting;
 using Microsoft.UI.Xaml;
-using Windows.ApplicationModel;
 
 // To learn more about WinUI, the WinUI project structure,
 // and more about our project templates, see: http://aka.ms/winui-project-info.