[submodule][201911] Update sonic-snmpagent (#6354)

b8f0c3a [snmpagent] [201911] Fix hardcoded qsfp lane count by reading sensor status from DB (sonic-net#183)

**- Why I did it**

Update submodule pointer for snmpagent to include fix for hardcoded qsfp lane count

**- How I did it**

Update snmpagent submodule

**- How to verify it**

Run build.