Make root-pools-test compile without warnings when APR_HAS_THREADS
is not defined.

* subversion/tests/libsvn_subr/root-pools-test.c
  (thread_func): hide completely if we don't support threads

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1527215 13f79535-47bb-0310-9956-ffa450edef68