Declare enum type to use from C++

Anonymous enum can't be referenced from C++. to fix this issue, declare enum
bt_iter_pos_type and use this type in struct bt_iter_pos.

Signed-off-by: Francis Giraldeau <francis.giraldeau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>