Fix build on OpenBSD caused by erroneous inclusion in allowed OS list…

… for sendmmsg() / recvmmsg() in nix-rust#1208.

In nix-rust#1208, sendmmsg() / recvmmsg() were added, but OpenBSD(who doesn't support these)
was included on the list of allowed operating systems for sendmmsg() related things.
This broke the build on OpenBSD.

For more Rust-world examples, see: rust-lang/libc@6f62973

src/sys/socket/mod.rs

@@ -920,7 +920,6 @@ pub fn sendmsg(fd: RawFd, iov: &[IoVec<&[u8]>], cmsgs: &[ControlMessage],
     target_os = "linux",
     target_os = "android",
     target_os = "freebsd",
-    target_os = "openbsd",
     target_os = "netbsd",
 ))]
 #[derive(Debug)]
@@ -956,7 +955,6 @@ pub struct SendMmsgData<'a, I, C>
     target_os = "linux",
     target_os = "android",
     target_os = "freebsd",
-    target_os = "openbsd",
     target_os = "netbsd",
 ))]
 pub fn sendmmsg<'a, I, C>(

test/sys/test_socket.rs

@@ -309,7 +309,6 @@ mod recvfrom {
         target_os = "linux",
         target_os = "android",
         target_os = "freebsd",
-        target_os = "openbsd",
         target_os = "netbsd",
     ))]
     #[test]