PR51307: Fix ldap_rebind build error when ldap is enabled and APR is built without threads.

Submitted By: Philupp <pippo deo.li>
Reviewed By: covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1130186 13f79535-47bb-0310-9956-ffa450edef68