Add header

Author: arcadiogarcia

Microsoft.Toolkit.Uwp.UI.Controls.Core/DropShadowPanel/IAlphaMaskProvider.cs

@@ -1,3 +1,7 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
 using Windows.UI.Composition;
 
 /// <summary>