chore: exclude secp256k1 from codespell

[JeneaVranceanu]

Summary of Changes

Current CI fails with codespell checks in secp256k1 directory.

Test Data or Screenshots

By submitting this pull request, you are confirming the following:

• I have reviewed the Contribution Guidelines.
• I have performed a self-review of my own code.
• I have updated my repository to match the develop branch.
• I have included test data or screenshots that prove my fix is effective or that my feature works.
• I have checked that all tests work and swiftlint is not throwing any errors/warnings.