Add missing header

Author: michael-hawker

components/Adorners/src/Adorner.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 CommunityToolkit.WinUI.Helpers;
 
 namespace CommunityToolkit.WinUI;