add param $flags to method getMulti in couchbase-api.php

Source: https://github.com/couchbase/php-ext-couchbase/pull/6
Change-Id: I6da6b4a7061c306813d716367c786b1e81839552
Reviewed-on: http://review.couchbase.org/25849
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>