Update lcb_create_io_ops_st layout in lcb_create_io_ops(3)

Change-Id: I223ffd63422c0b3bcc9b87140280102336e8c3c7
Reviewed-on: http://review.couchbase.org/28014
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>