Make the secp256k1_declassify argument constant

Summary:
This is required to declassify pointers to constant memory. Declassify should
never modify its argument.

This is a partial backport of secp256k1 PR558 : https://github.com/bitcoin-core/secp256k1/pull/558/commits/3e08b02e2a78f2a1fc457efab665db8ab8085373

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7636