ACPI / PM: Fix build problem related to acpi_target_system_state()

ACPI / PM: Fix build problem related to acpi_target_system_state()

Commit b87b49cd0efd ("ACPI / PM: Move device PM functions related to sleep
states") declared acpi_target_system_state() for CONFIG_PM_SLEEP whereas
it is only defined for CONFIG_ACPI_SLEEP, resulting in the following link
error:

drivers/built-in.o: In function `acpi_pm_device_sleep_wake':
drivers/acpi/device_pm.c:342: undefined reference to `acpi_target_system_state'
drivers/built-in.o: In function `acpi_dev_suspend_late':
drivers/acpi/device_pm.c:501: undefined reference to `acpi_target_system_state'
drivers/built-in.o: In function `acpi_pm_device_sleep_state':
drivers/acpi/device_pm.c:221: undefined reference to `acpi_target_system_state'

Define it only for CONFIG_ACPI_SLEEP and fallback to a dummy definition
for other configs.

[rjw: The problem only occurs for exotic .configs in which
 HIBERNATE_CALLBACKS is selected by XEN_SAVE_RESTORE and neither
 SUSPEND nor HIBERNATION is set.]

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>