Add license header to OpenAL example

Author: Exanite

examples/CSharp/OpenAL/Tutorial001.HelloSound/Program.cs

@@ -1,4 +1,7 @@
-using NAudio.Wave;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using NAudio.Wave;
 using Silk.NET.OpenAL;
 
 // NOTE: If you prefer, you can use the static ALContext and AL functions instead of creating an API object!