netinet/in.h: add IPV6_ROUTER_ALERT_ISOLATE from linux v5.1

restricts router alert packets received by the socket to the
socket's namespace only. see

  linux commit 9036b2fe092a107856edd1a3bad48b83f2b45000
  net: ipv6: add socket option IPV6_ROUTER_ALERT_ISOLATE

include/netinet/in.h

@@ -336,6 +336,7 @@ struct ip6_mtuinfo {
 #define IPV6_JOIN_ANYCAST       27
 #define IPV6_LEAVE_ANYCAST      28
 #define IPV6_MULTICAST_ALL      29
+#define IPV6_ROUTER_ALERT_ISOLATE 30
 #define IPV6_IPSEC_POLICY       34
 #define IPV6_XFRM_POLICY        35
 #define IPV6_HDRINCL            36