iio: magnetometer: Only declare ACPI table when ACPI is enable

Don't inflate the kernel size with data that isn't used. The conditional
declaration also fixes the following warning when building with clang:

drivers/iio/magnetometer/ak8975.c:704:36: error: variable 'ak_acpi_match'
  is not needed and will not be emitted
  [-Werror,-Wunneeded-internal-declaration]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/magnetometer/ak8975.c

@@ -784,6 +784,7 @@ static const struct iio_info ak8975_info = {
 	.driver_module = THIS_MODULE,
 };
 
+#ifdef CONFIG_ACPI
 static const struct acpi_device_id ak_acpi_match[] = {
 	{"AK8975", AK8975},
 	{"AK8963", AK8963},
@@ -793,6 +794,7 @@ static const struct acpi_device_id ak_acpi_match[] = {
 	{ },
 };
 MODULE_DEVICE_TABLE(acpi, ak_acpi_match);
+#endif
 
 static const char *ak8975_match_acpi_device(struct device *dev,
 					    enum asahi_compass_chipset *chipset)