Added missing header

Author: andrewleader

Microsoft.Toolkit.Uwp.Notifications/Toasts/IToastActivateableBuilder.cs

@@ -1,6 +1,8 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
+// 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;
 
 namespace Microsoft.Toolkit.Uwp.Notifications
 {