Follow-up to r1540590: Fix build with pre-1.3 APR.

* subversion/libsvn_subr/object_pool.c
  (): #include APR compatibility header


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540591 13f79535-47bb-0310-9956-ffa450edef68