ACPI: scan: fix undeclared variable warnings by including sleep.h

There are two pieces of data being exported from drivers/acpi/scan.c
(acpi_device_lock and acpi_wakeup_device_list) that don't have their
definitions declared in anything scan.c is including.

Fix the following sparse warnings by including sleep.h to add the
declarations of acpi_device_lock and acpi_wakeup_device_list to
fix the followng sparse warnings:

drivers/acpi/scan.c:42:1: warning: symbol 'acpi_device_lock' was not declared. Should it be static?
drivers/acpi/scan.c:43:1: warning: symbol 'acpi_wakeup_device_list' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>