ir: fields.c: fix public function names to get/set struct. field

ir: fields.c: fix public function names to get/set struct. field

To remain symmetric, use bt_ctf_field_structure_get_field_by_name() and
bt_ctf_field_structure_set_field_by_name(), as it is possible that
bt_ctf_field_structure_set_field_by_index() exists in the future.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>