Fix git remote callbacks

Author: jairbubbles

LibGit2Sharp/Core/GitRemoteCallbacks.cs

@@ -33,6 +33,8 @@ internal struct GitRemoteCallbacks
 
         internal IntPtr transport;
 
+        private IntPtr padding; // TODO: add git_remote_ready_cb
+
         internal IntPtr payload;
 
         internal NativeMethods.url_resolve_callback resolve_url;