ospfd: Remove unused function

ospfd: Remove unused function

The ospf_external_route_lookup function was not
being used so let's just remove it.

Unfortunately the removal was not quite so simple as
that ospf_asbr.h was being used to generate a reference
for the `struct ospf_route` data structure, so we
need to fix up the compile by fixing up header
inclusions so that ospf_route.h is actually included

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>