Add missing copyright note in ClampVector4

tests/ImageSharp.Benchmarks/General/BasicMath/ClampVector4.cs

@@ -1,8 +1,9 @@
-using System;
+// Copyright (c) Six Labors and contributors.
-using System.Collections.Generic;
+// Licensed under the Apache License, Version 2.0.
+
 using System.Numerics;
 using System.Runtime.CompilerServices;
-using System.Text;
+
 using BenchmarkDotNet.Attributes;
 
 namespace SixLabors.ImageSharp.Benchmarks.General.BasicMath