Added missing header text to BokehBlurKernelDataProvider.cs

Author: Sergio0694

src/ImageSharp/Processing/Processors/Convolution/Parameters/BokehBlurKernelDataProvider.cs

@@ -1,3 +1,6 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;