add ETH_P_PREAUTH ethertype from linux v4.17

added in linux commit 4fe0de5b143762d327bfaf1d7be7c5b58041a18c

include/netinet/if_ether.h

@@ -56,6 +56,7 @@
 #define ETH_P_8021AD	0x88A8
 #define ETH_P_802_EX1	0x88B5
 #define ETH_P_ERSPAN	0x88BE
+#define ETH_P_PREAUTH	0x88C7
 #define ETH_P_TIPC	0x88CA
 #define ETH_P_MACSEC	0x88E5
 #define ETH_P_8021AH	0x88E7