[secp256k1] Pass the SECP256K1_BUILD flag when building secp256k1's tooling.

Summary: As per title. This is actualy required.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1274