net: rpl: Repair root functions are made global

The net_rpl_repair_root() and net_rpl_set_root_with_version()
functions were static which prevented global repair when using
an instance id. Making those functions global allows RPL network
repairing from shell for example.

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