[sonic-snmpagent] Update submodule (sonic-net#9076)

#### Why I did it
Includes below commits
```
c2d4945 2021-09-17 | [snmp] Allow system with no ports in config db run without errors (#221) [Lior Avramov]
fccb21b 2021-08-30 | [RFC1213]: Initialize lag oid map in reinit_data instead of (#232) [SuvarnaMeenakshi]
```