default_engine: Fix DEBUG_REFCNT macro

Change-Id: I5b9de60993f49d50cd3fb514eb68a410c69d447f
Reviewed-on: http://review.couchbase.org/44713
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>