Follow-up to r1555491: Apply a similar patch to FSX.

* subversion/libsvn_fs_x/tree.c
  (make_txn_root): Use the correct pool.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555625 13f79535-47bb-0310-9956-ffa450edef68