Make the type of the second argument to a DATFunc_heur_table explicit.

It's always pased a heur_dissector_list_t *, so give it that type,
rather than having it be a generic pointer.

Change-Id: Ia6a045bb1b96c2f6ef3e23f27928e0b52f7cfb9f
Reviewed-on: https://code.wireshark.org/review/5713
Reviewed-by: Guy Harris <guy@alum.mit.edu>