added destroy_mutex to non-win32 portion

Submitted by:	Ben Hyde <bhyde@gensym.com>
Reviewed by:	Dean Gaudet, Jim Jagielski


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79786 13f79535-47bb-0310-9956-ffa450edef68