Added missing using

Author: mdupras

src/ImageSharp.Web/Middleware/ImageSharpMiddleware.cs

@@ -9,6 +9,7 @@
 using Microsoft.Extensions.Options;
 using Microsoft.IO;
 using SixLabors.ImageSharp.Formats;
+using SixLabors.ImageSharp.PixelFormats;
 using SixLabors.ImageSharp.Web.Caching;
 using SixLabors.ImageSharp.Web.Commands;
 using SixLabors.ImageSharp.Web.Processors;