fix compiling with boost 1.76; add <set> include in chainbase

[spoonincode]

Change Description

Needed this to compile with boost 1.76 beta. chainbase is using std::multiset so needs to include <set>

Needs EOSIO/chainbase#74

Change Type

Select ONE:

• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

Select ANY that apply:

• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions