ACPI: suspend: build-fix for CONFIG_SUSPEND=n and CONFIG_HIBERNATION=y

This fixes compilation with CONFIG_SUSPEND unset and CONFIG_HIBERNATION set
(raf. http://marc.info/?l=linux-acpi&m=119055289723895&w=4

).

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>