Fix some compile problems on weird/old compilers.

Summary:
The visibility attribute is a GCC 4+ feature.
GCC 2.95 also warns about the unsigned/signed comparision.

This is a backport of libsecp256k1 PR773

Test Plan:

ninja check-secp256k1

I do not have GCC 2.95 installed, so I'll trust this is fixing the problem.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7594