Added missing file headers

Author: Sergio0694

Microsoft.Toolkit.Mvvm/Messaging/Internals/Type2.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.Runtime.CompilerServices;
 
 namespace Microsoft.Toolkit.Mvvm.Messaging.Internals

Microsoft.Toolkit.Mvvm/Messaging/Internals/Unit.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.Runtime.CompilerServices;
 
 namespace Microsoft.Toolkit.Mvvm.Messaging.Internals