Add ip_mreqn on Apple platforms

Available since macOS 10.7. See
https://developer.apple.com/documentation/kernel/ip_mreqn.

Author: tamird

libc-test/semver/apple.txt

@@ -1724,6 +1724,7 @@ in6_pktinfo
 in_pktinfo
 initgroups
 integer_t
+ip_mreqn
 ipc_perm
 kern_return_t
 kevent

src/unix/bsd/apple/mod.rs

@@ -215,6 +215,12 @@ s! {
         pub imr_interface: in_addr,
     }
 
+    pub struct ip_mreqn {
+        pub imr_multiaddr: in_addr,
+        pub imr_address: in_addr,
+        pub imr_ifindex: ::c_int,
+    }
+
     pub struct aiocb {
         pub aio_fildes: ::c_int,
         pub aio_offset: ::off_t,