Add missing using

Author: Kronicaler

src/dotnet-grpc/Commands/AddFileCommand.cs

@@ -19,6 +19,7 @@
 using System;
 using System.CommandLine;
 using System.CommandLine.Invocation;
+using System.CommandLine.Parsing;
 using Grpc.Dotnet.Cli.Internal;
 using Grpc.Dotnet.Cli.Options;
 using Grpc.Dotnet.Cli.Properties;