[submodule]: update sonic-sairedis

* 27a93ff 2019-11-25 | Don't learn fdb entries on LAG when rif based (sonic-net#538) [Kamil Cudnik]
* 5ef1764 2019-11-22 | Changes in sonic-sairedis repo to support the NAT feature. (sonic-net#519) [Kiran Kumar Kella]

Signed-off-by: Guohan Lu <[email protected]>

Author: lguohan