Add a heur_dissector_table_foreach() function.

This is, for heuristic dissector tables, the equivalent of
dissector_table_foreach() for keyed dissector tables.

Change-Id: I4b2f870e1c1179fda1adddd93930b83aaaaf8763
Reviewed-on: https://code.wireshark.org/review/5715
Reviewed-by: Guy Harris <guy@alum.mit.edu>