netinet/if_ether.h: add ETH_TLEN from linux v4.16

octets in ethernet type field
added in linux commit 4bbb3e0e8239f9079bf1fe20b3c0cb598714ae61

include/netinet/if_ether.h

@@ -5,6 +5,7 @@
 #include <sys/types.h>
 
 #define ETH_ALEN	6
+#define ETH_TLEN	2
 #define ETH_HLEN	14
 #define ETH_ZLEN	60
 #define ETH_DATA_LEN	1500