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>