[cmake] Fix a typo in the bench CMakeLists.txt

Summary: The dependency is already satisfied because the file is permanent, so there is no change in behavior except it removes a lot of cmake warnings.

Test Plan:

ninja bench-bitcoin

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17430