tor-rtcompat: Avoid wildcard dependency requirements.

This is preventing us from publishing tor-rtcompat:

error: failed to publish to registry at https://crates.io

Caused by: the remote server responded with an error (status 400 Bad Request): wildcard (*) dependency constraints are not allowed on crates.io. Crate with this problem: hex See https://doc.rust-lang.org/cargo/faq.html#can-libraries-use--as-a-version-for-their-dependencies for more information