Follow-up to r1619068.  Missed a conditionally compiled caller.

* subversion/libsvn_subr/cache-membuffer.c
  (svn_cache__membuffer_cache_create): Update svn_mutex__init caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619071 13f79535-47bb-0310-9956-ffa450edef68