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>