davautocheck: Prefork also needs the Mutex argument in 2.3.x+.

* subversion/tests/cmdline/davautocheck.sh: Use the Mutex directive for prefork


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544303 13f79535-47bb-0310-9956-ffa450edef68