Add logic to determine if a "tvb get" call is used as a parameter to a proto_tree_add_text call. convert_proto_tree_add_text.pl has an easier time determining hf_ field values with a "tvb get" call present, so it's more likely those dissectors will be picked first in eliminating their proto_tree_add_text calls.

Change-Id: I3dd57a73c8e60d8075a6bb987efd52b96b38445c
Reviewed-on: https://code.wireshark.org/review/4997
Reviewed-by: Michael Mann <mmann78@netscape.net>