ACPI / PM: Fix unused function warnings for CONFIG_PM_SLEEP

According to compiler warnings, several suspend/resume functions
in ACPI drivers are not used for CONFIG_PM_SLEEP unset, so add
#ifdefs to prevent them from being built in that case.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
diff --git a/drivers/acpi/battery.c b/drivers/acpi/battery.c
index ff2c876..45e3e17 100644
--- a/drivers/acpi/battery.c
+++ b/drivers/acpi/battery.c
@@ -1052,6 +1052,7 @@
 	return 0;
 }
 
+#ifdef CONFIG_PM_SLEEP
 /* this is needed to learn about changes made in suspended state */
 static int acpi_battery_resume(struct device *dev)
 {
@@ -1068,6 +1069,7 @@
 	acpi_battery_update(battery);
 	return 0;
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(acpi_battery_pm, NULL, acpi_battery_resume);