lib: Sort zapi_nexthops on the encode

lib: Sort zapi_nexthops on the encode

Sort nexthops before we push them to zebra. This offloads
the nexthop sorting zebra is doing onto the upper level protocols
so that when it gets to zebra and we construct a group, it just has
to append them to the tail for every nexthop.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>