Follow-up to 1568009: Re-add a line that was removed accidentally.

* subversion/libsvn_subr/cache-membuffer.c
  (drop_entry): We still need to track cache memory usage.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568072 13f79535-47bb-0310-9956-ffa450edef68