ACPI / PM: Export acpi_target_system_state() to modules

Export the acpi_target_system_state() function to modules so that
modular drivers can use it to check what the target ACPI sleep state
of the system is (that is needed for i915 mostly at this point).
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/sleep.c

@@ -89,6 +89,7 @@ u32 acpi_target_system_state(void)
 {
 	return acpi_target_sleep_state;
 }
+EXPORT_SYMBOL_GPL(acpi_target_system_state);
 
 static bool pwr_btn_event_pending;