platform / x86 / PM: Fix unused function warnings for CONFIG_PM_SLEEP

According to compiler warnings, quite some suspend/resume functions
in platform x86 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/platform/x86/xo15-ebook.c b/drivers/platform/x86/xo15-ebook.c
index 849c07c..38ba39d 100644
--- a/drivers/platform/x86/xo15-ebook.c
+++ b/drivers/platform/x86/xo15-ebook.c
@@ -77,10 +77,12 @@
 	}
 }
 
+#ifdef CONFIG_PM_SLEEP
 static int ebook_switch_resume(struct device *dev)
 {
 	return ebook_send_state(to_acpi_device(dev));
 }
+#endif
 
 static SIMPLE_DEV_PM_OPS(ebook_switch_pm, NULL, ebook_switch_resume);