Add codepages for .NET Core.

DiscImageChef.Gtk/Program.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Reflection;
+using System.Text;
 using System.Threading.Tasks;
 using DiscImageChef.Core;
 using DiscImageChef.Database;
@@ -58,6 +59,8 @@ namespace DiscImageChef.Gtk
                         break;
                 }
 
+            Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+
             new Application(Platforms.Gtk).Run(new frmMain(Debug, Verbose));
 
             Statistics.SaveStats();