ACPI: suppress compiler warnings in button.c

This patch fixes following compiler warnings when build via make W=1:

drivers/acpi/button.c:220:5: warning: no previous prototype for ‘acpi_lid_notifier_register’ [-Wmissing-prototypes]
drivers/acpi/button.c:226:5: warning: no previous prototype for ‘acpi_lid_notifier_unregister’ [-Wmissing-prototypes]
drivers/acpi/button.c:232:5: warning: no previous prototype for ‘acpi_lid_open’ [-Wmissing-prototypes]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/button.c

@@ -33,6 +33,7 @@
 #include <linux/slab.h>
 #include <acpi/acpi_bus.h>
 #include <acpi/acpi_drivers.h>
+#include <acpi/button.h>
 
 #define PREFIX "ACPI: "