update icmphdr struct following linux v4.8

add union field that is used in the kernel for SIT/GRE tunneling ICMPv4
messages. see linux commit 20e1954fe238dbe5f8d3a979e593fe352bd703cf

include/netinet/ip_icmp.h

@@ -23,6 +23,7 @@ struct icmphdr {
 			uint16_t __unused;
 			uint16_t mtu;
 		} frag;
+		uint8_t reserved[4];
 	} un;
 };