Add missing copyright header (#5020)

Author: MichaelSimons

src/Microsoft.TestPlatform.CoreUtilities/GlobalSuppressions.cs

@@ -1,4 +1,7 @@
-// This file is used by Code Analysis to maintain SuppressMessage
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+// This file is used by Code Analysis to maintain SuppressMessage
 // attributes that are applied to this project.
 // Project-level suppressions either have no target or are given
 // a specific target and scoped to a namespace, type, member, etc.