ACPI / button: fix button driver compile error when CONFIG_PM_SLEEP is undefined

The ACPI button driver defines acpi_button_resume() when
CONFIG_PM_SLEEP is defined. This results in the following
compile error when CONFIG_PM_SLEEP is undefined:

drivers/acpi/button.c:85:8: error: ‘acpi_button_resume’ undeclared here (not in a function)

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>