Add missing using directive

Author: mlorbetske

src/dotnet/commands/dotnet-run/LaunchSettings/LaunchSettingsManager.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using Microsoft.DotNet.Cli.Utils;
+using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 
 namespace Microsoft.DotNet.Tools.Run.LaunchSettings