net: rpl: Fix compilation if CONFIG_NET_RPL_MOP3 is enabled

If multicast routing is set meaning that MOP3 (Mode of Operation)
is enabled, then the code was not compiling properly.

Change-Id: Ice8a9f7b705c781536d3c2c5ca6cc2bb77a7acc1
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>