[CMAKE] Add an install target for the secp256k1 benchmarks

Summary: The secp256k1 benchmarks are missing an installation target.

Test Plan:

ninja install-secp256k1-bench

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7314