Add License Header

Microsoft.Toolkit.Uwp.UI/Extensions/DispatcherTimerExtensions.cs

@@ -1,4 +1,8 @@
-using System;
+// 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 System;
 using System.Collections.Concurrent;
 using Windows.UI.Xaml;