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>

include/linux/cpuidle.h

@@ -60,6 +60,8 @@ struct cpuidle_state {
 #define CPUIDLE_DRIVER_FLAGS_MASK (0xFFFF0000)
 
 struct cpuidle_device_kobj;
+struct cpuidle_state_kobj;
+struct cpuidle_driver_kobj;
 
 struct cpuidle_device {
 	unsigned int		registered:1;