Fixup dtrace build of default_engine

Change-Id: Ib2af3784c130d9d8e951aebbc942c789e9d1f0a9
Reviewed-on: http://review.couchbase.org/40131
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>