cpuidle: Add missing forward declarations of structures

Add missing forward declarations of struct cpuidle_state_kobj and
struct cpuidle_driver_kobj in cpuidle.h.

[rjw: Changelog]
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>