Explicitly include api3.h in iotests

Fixes build failure

Change-Id: Ia63bf706999551d6b9024718ec4eaadee47a94ca
Reviewed-on: http://review.couchbase.org/41503
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>