core: Make ostree_set_xattrs() private

Nothing external uses it.  We keep ostree_get_xattrs_for_file() public
because it's convenient for external consumers to get xattrs in
exactly the format we desire.

https://bugzilla.gnome.org/show_bug.cgi?id=707733

src/libostree/ostree-repo-checkout.c

@@ -217,7 +217,7 @@ create_file_from_input (GFile            *dest_file,
 
   if (xattrs != NULL)
     {
-      if (!ostree_set_xattrs (dest_file, xattrs, cancellable, error))
+      if (!_ostree_set_xattrs (dest_file, xattrs, cancellable, error))
         goto out;
     }