[CMAKE] Optionally install bitcoin-bench

Summary: As per title.

Test Plan:
Won't install bitcoin-bench:

ninja install

Will install bitcoin-bench:

cmake -DCOMPONENT=bitcoin-bench -P cmake_install.cmake

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6960