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-commit.c

@@ -91,7 +91,7 @@ commit_loose_object_trusted (OstreeRepo        *self,
        */
       if (xattrs != NULL)
         {
-          if (!ostree_set_xattrs (temp_file, xattrs, cancellable, error))
+          if (!_ostree_set_xattrs (temp_file, xattrs, cancellable, error))
             goto out;
         }
     }