Add missing using statement

Author: nblumhardt

src/Serilog.AspNetCore/SerilogWebHostBuilderExtensions.cs

@@ -1,4 +1,4 @@
-// Copyright 2017-2019 Serilog Contributors
+// Copyright 2017-2019 Serilog Contributors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Extensions.Logging;
+using Serilog.Core;
 using Serilog.Extensions.Hosting;
 using Serilog.Extensions.Logging;