apr_os_thread_current() call needs #if APR_HAS_THREADS protection

PR:
Obtained from:
Submitted by:    Justin Erenkrantz <jerenkrantz@ebuilt.com>
Reviewed by:


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90622 13f79535-47bb-0310-9956-ffa450edef68