ldpd: allow targeted neighbors over any interface

ldpd: allow targeted neighbors over any interface

It's doesn't make sense to enforce that a targeted-hello is received
on an LDP-enabled interface. It should be possible, for example, to use
LDP only to signal pseudowires and other another protocol (e.g. RSVP-TE)
to create end-to-end LSPs.

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