Fix building libstd documentation on FreeBSD.
It fixes the following 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`
```
|
||
|---|---|---|
| .. | ||
| benches | ||
| primitive_docs | ||
| src | ||
| tests | ||
| build.rs | ||
| Cargo.toml | ||