Fix rpl-udp build failure on native target, reported on contiki mailing list
Fix rpl-udp build failure on native target, reported on contiki mailing list

http://sourceforge.net/mailarchive/forum.php?thread_name=a2b9cb98ac4a191867afc953113fabaa%40amnesiak.org&forum_name=contiki-developers

MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,arurke/contiki,arurke/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,arurke/contiki