[build] link boost

Summary: In D13966 I broke boost linking (build-native-osx failed on CI) when removing thread. Something needs to be linked for boost to work.

Test Plan:
cmake .. -GNinja && ninja check

native osx build on CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13969