Added struct ip_mreq (#143)

libc/sock/sock.h

@@ -53,6 +53,12 @@ struct sockaddr_storage {
   };
 };
 
+struct ip_mreq  {
+  struct in_addr imr_multiaddr;   /* IP multicast address of group */
+  struct in_addr imr_interface;   /* local IP address of interface */
+};
+
+
 struct pollfd {
   int32_t fd;
   int16_t events;