uip-icmp6.c: call rpl_insert_header only when UIP_CONF_IPV6_RPL is set
uip-icmp6.c: call rpl_insert_header only when UIP_CONF_IPV6_RPL is set

arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki