yang: fix circular chain of leafrefs

yang: fix circular chain of leafrefs

Fix the following libyang error when trying to load the "frr-nexthop"
module explicitly (e.g. using the 'gen_northbound_callbacks' tool):

libyang: A circular chain of leafrefs detected. (/frr-nexthop:frr-nexthop-group/nexthop-groups/name)
libyang: Invalid value "frr-nexthop-grouping" of "uses". (/frr-nexthop:frr-nexthop-group/frr-nexthop-grouping)
libyang: Copying data from grouping failed. (/frr-nexthop:frr-nexthop-group/frr-nexthop-grouping)
libyang: Module "frr-nexthop" parsing failed.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>