gpio: gpio-mlxbf: Tell the compiler that ACPI functions may not be used

... as is the case when !CONFIG_ACPI.

Fixes the following W=1 kernel build warning:

 drivers/gpio/gpio-mlxbf.c:130:36: warning: ‘mlxbf_gpio_acpi_match’ defined but not used [-Wunused-const-variable=]
 130 | static const struct acpi_device_id mlxbf_gpio_acpi_match[] = {
 | ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Cc: Shravan Kumar Ramani <sramani@mellanox.com>
Link: https://lore.kernel.org/r/20200630133345.2232932-9-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>