Fixed Colors namespace in WinUI2

Author: Avid29

components/GradientSlider/src/GradientSlider.Input.cs

@@ -2,7 +2,11 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+#if !WINAPPSDK
+using Windows.UI;
+#else
 using Microsoft.UI;
+#endif
 
 namespace CommunityToolkit.WinUI.Controls;