netinet/in.h: add new IPV6_FREEBIND from linux v4.15

new socekt option for AF_INET6 SOL_RAW sockets, added in linux commit
84e14fe353de7624872e582887712079ba0b2d56

include/netinet/in.h

@@ -363,6 +363,7 @@ struct ip6_mtuinfo {
 #define IPV6_TRANSPARENT        75
 #define IPV6_UNICAST_IF         76
 #define IPV6_RECVFRAGSIZE       77
+#define IPV6_FREEBIND           78
 
 #define IPV6_ADD_MEMBERSHIP     IPV6_JOIN_GROUP
 #define IPV6_DROP_MEMBERSHIP    IPV6_LEAVE_GROUP