Minor type consistency fix.

* subversion/libsvn_subr/cache-membuffer.c
  (last_access_key_t): APR defines key sizes as signed, so let's do the same

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543601 13f79535-47bb-0310-9956-ffa450edef68