Bump copyright headers in bench

Summary:
Original script:

-BEGIN VERIFY SCRIPT-
./contrib/devtools/copyright_header.py update ./src/bench/
./contrib/devtools/copyright_header.py update ./src/test/
-END VERIFY SCRIPT-

Partial backport of core PR15788:
https://github.com/bitcoin/bitcoin/pull/15788/commits/faf400077d0144363c2dd24187431d958fb149a9

Bench files only for ease of review.

Test Plan: Check nothing but core's copyright year changed.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5495