[sonic-snmpagent] Update submodule (#8349)

Includes below commits:
```
43b5e1a 2021-08-04 | CPU Spike because of redundant and flooded keyspace notifis handled (#230) [Vivek Reddy]
4d6bb79 2021-08-02 | Non-block reading counters to tolerate corrupted/delayed counters in COUNTERS_DB (#229) [Qi Luo]
```