esp32: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register all root pointers in the
esp32 port.

Signed-off-by: David Lechner <david@pybricks.com>

ports/esp32/machine_timer.c

@@ -272,3 +272,5 @@ const mp_obj_type_t machine_timer_type = {
     .make_new = machine_timer_make_new,
     .locals_dict = (mp_obj_t)&machine_timer_locals_dict,
 };
+
+MP_REGISTER_ROOT_POINTER(struct _machine_timer_obj_t *machine_timer_obj_head);