fix(openpgp): make sequoia compile on wasm

Hi 👋🏻,

I tried to compile sequoia-openpgp in a WASM project, but it fails at these lines: openpgp/src/lib.rs#111

The chrono::Utc::now() function requires the clock feature as shown in their docs docs.rs/chrono/offset/Utc

I've temporarily fixed the issue for my part, but this MR can be useful.

Thanks