PM / hibernate: export hibernation_set_ops

To support the ability to implement PM hibernation code as modules
the hibernation_set_ops function requires to be exported.

Similar solution already available for suspend_set_ops
(please refer to commit a5e4fd87).

Signed-off-by: Leonardo Potenza <leonardo.potenza@intel.com>
Signed-off-by: Edwin Verplanke <edwin.verplanke@intel.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>