mfd: axp20x-i2c: Do not define 'struct acpi_device_id' when !CONFIG_ACPI

Since ACPI_PTR() is used to NULLify the value when !CONFIG_ACPI,
struct axp20x_i2c_acpi_match becomes defined by unused.

This squashes the current W=1 kernel builds warning:

 drivers/mfd/axp20x-i2c.c:82:36: warning: ‘axp20x_i2c_acpi_match’ defined but not used [-Wunused-const-variable=]

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Carlo Caione <carlo@caione.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/axp20x-i2c.c

@@ -79,6 +79,7 @@ static const struct i2c_device_id axp20x_i2c_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, axp20x_i2c_id);
 
+#ifdef CONFIG_ACPI
 static const struct acpi_device_id axp20x_i2c_acpi_match[] = {
 	{
 		.id = "INT33F4",
@@ -87,6 +88,7 @@ static const struct acpi_device_id axp20x_i2c_acpi_match[] = {
 	{ },
 };
 MODULE_DEVICE_TABLE(acpi, axp20x_i2c_acpi_match);
+#endif
 
 static struct i2c_driver axp20x_i2c_driver = {
 	.driver = {