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

lang/rust/files/patch-library_std_src_os_unix_net_ancillary.rs

@@ -0,0 +1,27 @@
+From 513f28db0a540725bc41f080a0f52973af67c3db Mon Sep 17 00:00:00 2001
+From: MikaelUrankar <mikael.urankar@gmail.com>
+Date: Tue, 13 Jun 2023 13:14:06 +0200
+Subject: [PATCH] Fix building the 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`
+---
+ library/std/src/os/unix/net/ancillary.rs | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/library/std/src/os/unix/net/ancillary.rs b/library/std/src/os/unix/net/ancillary.rs
+index 7565fbc0d099c..814f1c7c2838e 100644
+--- library/std/src/os/unix/net/ancillary.rs
++++ library/std/src/os/unix/net/ancillary.rs
+@@ -17,6 +17,7 @@ mod libc {
+     pub use libc::c_int;
+     pub struct ucred;
+     pub struct cmsghdr;
++    pub struct sockcred2;
+     pub type pid_t = i32;
+     pub type gid_t = u32;
+     pub type uid_t = u32;