Add missing prototype

src/include/gpxe/arp.h

 extern int arp_process ( struct net_interface *arp_netif,
 			 struct pk_buff *pkb );
 
+extern int arp_add_generic_header ( struct net_interface *arp_netif,
+				    struct pk_buff *pkb ) {
+
 #endif /* _ARP_H */