Added missing source file header

components/TitleBar/src/WndProcHelper.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.
+
 #if WINAPPSDK
 using System.Runtime.InteropServices;
 using WinRT.Interop;