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
Added the const keyword to IP address arguments that are not (and should not be) changed by the callee

i# const rpl-dag.c, fixup