[sonic-swss-common] Update submodule (#6197)

Including commits in sonic-swss-common repo:
```
40e695d 2020-12-12 | [pyext] Implement DBConnector::pubsub() to emulate python Redis.pubsub() (#427) [Qi Luo]
9a2bf6c 2020-12-12 | Add fdb_flush.v2.lua script to makefile (#425) [Kamil Cudnik]
7d60e7e 2020-12-07 | Handle exception in Select (#424) [Qi Luo]
```