add missing if_arp.h constant

ARPHRD_6LOWPAN (IPv6 over LoWPAN) added in
linux 3.14 commit 0abc652c796dab74d34d60473ec5594cd21620be

include/net/if_arp.h

@@ -91,6 +91,7 @@ struct arphdr {
 #define ARPHRD_CAIF 822
 #define ARPHRD_IP6GRE 823
 #define ARPHRD_NETLINK 824
+#define ARPHRD_6LOWPAN 825
 
 #define ARPHRD_VOID	  0xFFFF
 #define ARPHRD_NONE	  0xFFFE