gpio: gpio-mlxbf2: Tell the compiler that ACPI functions may not be use

... as is the case when !CONFIG_ACPI.

Fixes the following W=1 kernel build warning:

 drivers/gpio/gpio-mlxbf2.c:312:36: warning: ‘mlxbf2_gpio_acpi_match’ defined but not used [-Wunused-const-variable=]
 312 | static const struct acpi_device_id mlxbf2_gpio_acpi_match[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Asmaa Mnebhi <asmaa@mellanox.com>
Cc: Asmaa Mnebhi <Asmaa@mellanox.com>
Link: https://lore.kernel.org/r/20200630133345.2232932-10-lee.jones@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>