Fix warning on 'set but not used' variable
Fix warning on 'set but not used' variable

- if _LATENCY_STATS are not defined, compiler reports a warning
message.

Change-Id: I5e10bb990275d0a51cd6e6fafca179690e37791a

wurikiji/forestdb,couchbase/forestdb,hisundar/forestdb,hisundar/forestdb,wurikiji/forestdb,couchbase/forestdb