ACPI / button: fix defined but not used warning

Fix a build warning in the ACPI button driver when CONFIG_PROC_FS
is not enabled by marking the unused function as __maybe_unused.

../drivers/acpi/button.c:252:12: warning: 'acpi_button_state_seq_show' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/button.c

@@ -21,6 +21,7 @@
 
 #define pr_fmt(fmt) "ACPI: button: " fmt
 
+#include <linux/compiler.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/init.h>
@@ -249,7 +250,8 @@ static int acpi_lid_notify_state(struct acpi_device *device, int state)
 	return ret;
 }
 
-static int acpi_button_state_seq_show(struct seq_file *seq, void *offset)
+static int __maybe_unused acpi_button_state_seq_show(struct seq_file *seq,
+						     void *offset)
 {
 	struct acpi_device *device = seq->private;
 	int state;