Commit e5cc8ef3 (ACPI / PM: Provide ACPI PM callback routines for
subsystems) introduced a build problem occuring if CONFIG_ACPI is
unset or CONFIG_PM is unset and errno.h is not included before
acpi.h, because in that case ENODEV used in acpi.h is undefined.

Fix the issue by making acpi.h include errno.h.
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>