Register error-chain cfg for rustc checking.

[Randy808]

Summary

Register has_error_description_deprecated as a known Rust cfg.

error-chain 0.12.4 emits this cfg while expanding its macro in electrs. Recent Rust toolchains therefore report it as unexpected, even though it originates in
the dependency.

This narrowly registers that cfg instead of disabling the unexpected_cfgs lint. There are no runtime behavior changes.

Testing

• cargo check --locked
• cargo check --features liquid --locked

Both builds pass and no longer emit the has_error_description_deprecated warning.