PCI hotplug: Fix build with CONFIG_ACPI unset

One of the recent changes caused complilation of
drivers/pci/hotplug/pciehp_core.c to fail.  Fix this issue.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>