React to more ABI changes

Author: bording

LibGit2Sharp/Core/GitWorktree.cs

@@ -1,7 +1,5 @@
 using System;
-using System.Collections.Generic;
 using System.Runtime.InteropServices;
-using System.Text;
 
 namespace LibGit2Sharp.Core
 {
@@ -36,6 +34,8 @@ internal class git_worktree_add_options
 
         public int locked;
 
+        public int checkout_existing;
+
         public IntPtr @ref = IntPtr.Zero;
 
         public GitCheckoutOpts checkoutOpts = new GitCheckoutOpts { version = 1 };