Fix: unused-function warnings in lib/graph/iterator.c

Fix: unused-function warnings in lib/graph/iterator.c

clang warnings:
  iterator.c:87:6: error: unused function '_set_self_comp_port_input_msg_iterator_state' [-Werror,-Wunused-function]
  void _set_self_comp_port_input_msg_iterator_state(
       ^
  iterator.c:885:6: error: unused function '_set_iterator_state_after_seeking' [-Werror,-Wunused-function]
  void _set_iterator_state_after_seeking(
       ^

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ia409f1dd696cdfed58de23aad926c4effdc26307
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1034
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>