Add sw_if_index to the ip_neighbor_details_t response.

When a DUMP with sw_if_index == ~0 is used to get all Neighbor
entries for all interfaces, it is unclear in the details to
which interface the neighbor belongs.

Clear that up by returning the associated sw_if_index as well.

Change-Id: Ib584a57138f7faceffed64d7c1854f7af92e0e42
Signed-off-by: Jon Loeliger <jdl@netgate.com>