[build] don't link boost::filesystem

Summary: This was missed in D12608, and the cross-compilation builds are not happy about it.

Test Plan: build-win64 build-osx build-linux64 gitian-osx gitian-linux gitian-win

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13754