Small compatibility improvement. No functional change.

* subversion/libsvn_subr/cache-membuffer.c
  (get_group_index,
   combine_key): use APR_UINT64_C() instead of ull suffix

Patch by: joeswatosh

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1490221 13f79535-47bb-0310-9956-ffa450edef68