multimedia/ringrtc: Fix build with rust 1.71.0

Do not abort if documentation is missing as we don't need it.

error: missing documentation for an extern crate

--> curve25519-dalek-4f0aa6653c51598daa0a2f53b8ba54ce0eedfbdd/src/lib.rs:265:1
 |

265 | pub extern crate digest;

| ^^^^^^^^^^^^^^^^^^^^^^^

PR: 272449