Add missing license header

Author: flobernd

src/Elastic.Clients.Elasticsearch/_Shared/Types/Cluster/WaitForNodes.cs

@@ -1,3 +1,7 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information.
+
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;