[SECP256K1] Add parens around ROUND_TO_ALIGN's parameter.
[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 [[https://github.com/bitcoin-core/secp256k1/pull/880 | 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

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