Rust libc does not provide ip_mreqn for openbsd

The OS proper, though, supports it.

Author: niklasha

src/sys/unix.rs

@@ -1004,6 +1004,7 @@ pub(crate) fn from_in6_addr(addr: in6_addr) -> Ipv6Addr {
     target_os = "haiku",
     target_os = "illumos",
     target_os = "netbsd",
+    target_os = "openbsd",
     target_os = "redox",
     target_os = "solaris",
 )))]