[submodule]: update sonic-snmpagent

* 3885f19 2021-02-02 | let sysDescr_pass use python3 (sonic-net#195) (HEAD, origin/master, origin/HEAD) [lguohan]
* ad302d4 2021-01-28 | Fix: 'key not found' exception in bgp4.py (sonic-net#192) [bingwang-ms]
* 59e2a1c 2021-01-27 | Fix fdb_vlanmac() on corner cases (sonic-net#193) [Qi Luo]

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

Author: lguohan