[SECP256K1] Remove underscores from header defs.

Summary:

This makes them consistent with other files and avoids reserved
identifiers.

Backport of secp256k1#874

Applied to multiset and schnorr as well.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9385