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
Merge pull request #322 from ismb-pert/fix-rpd-udp

Fix build failure on rpl-udp reported on contiki mailing list