Expose find_heur_dissector_list().

We already have find_dissector_table(); expose
find_heur_dissector_list() as well, so that heuristic dissector lists
can be shared.

Change-Id: I3f50413b8b10fd3129fcdc2344cb1447f0946ce9
Reviewed-on: https://code.wireshark.org/review/5701
Reviewed-by: Guy Harris <guy@alum.mit.edu>