[SECP256K1] Add parens around ROUND_TO_ALIGN's parameter.

Summary:

This makes the macro robust against a hypothetical ROUND_TO_ALIGN(foo ?
sizeA : size B) invocation.

Backport of secp256k1#880

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9387