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

drivers/gpio/gpio-mlxbf2.c

@@ -309,7 +309,7 @@ static int mlxbf2_gpio_resume(struct platform_device *pdev)
 }
 #endif
 
-static const struct acpi_device_id mlxbf2_gpio_acpi_match[] = {
+static const struct acpi_device_id __maybe_unused mlxbf2_gpio_acpi_match[] = {
 	{ "MLNXBF22", 0 },
 	{},
 };