lang/rust: Fix docs building.

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`

Reported by: kib
Differential Revision: https://reviews.freebsd.org/D40509