[secp256k1] Fix bench_internal build

Summary: bench_internal try to include hash.h when the multiset module is enabled. However, this file is not in the include path, so we add it there.

Test Plan:

make -C src/secp256k1 bench_internal

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel

Differential Revision: https://reviews.bitcoinabc.org/D1278