lang/rust: Fix docs building.

[mikael]

Summary

Add the missing "pub struct sockcred2" in module "libc".
Error:

error[E0412]: cannot find type `sockcred2` in module `libc`
   --> library/std/src/os/unix/net/ancillary.rs:211:29
    |
211 | pub struct SocketCred(libc::sockcred2);
    |                             ^^^^^^^^^ not found in `libc`

Upstream PR: https://github.com/rust-lang/rust/pull/112579
Reported by: kib