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>

include/acpi/acpi_bus.h

@@ -470,11 +470,9 @@ static inline int acpi_pm_device_run_wake(struct device *dev, bool enable)
 #endif
 
 #ifdef CONFIG_PM_SLEEP
-u32 acpi_target_system_state(void);
 int __acpi_device_sleep_wake(struct acpi_device *, u32, bool);
 int acpi_pm_device_sleep_wake(struct device *, bool);
 #else
-static inline u32 acpi_target_system_state(void) { return ACPI_STATE_S0; }
 static inline int __acpi_device_sleep_wake(struct acpi_device *adev,
 					   u32 target_state, bool enable)
 {
@@ -486,6 +484,12 @@ static inline int acpi_pm_device_sleep_wake(struct device *dev, bool enable)
 }
 #endif
 
+#ifdef CONFIG_ACPI_SLEEP
+u32 acpi_target_system_state(void);
+#else
+static inline u32 acpi_target_system_state(void) { return ACPI_STATE_S0; }
+#endif
+
 static inline bool acpi_device_power_manageable(struct acpi_device *adev)
 {
 	return adev->flags.power_manageable;