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

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