iio: light: isl29018: Only declare ACPI table when ACPI is enabled

This fixes the following warning when building with clang:

drivers/iio/light/isl29018.c:808:36: error: variable
    'isl29018_acpi_match' is not needed and will not be emitted
    [-Werror,-Wunneeded-internal-declaration]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/light/isl29018.c

@@ -807,13 +807,15 @@ static SIMPLE_DEV_PM_OPS(isl29018_pm_ops, isl29018_suspend, isl29018_resume);
 #define ISL29018_PM_OPS NULL
 #endif
 
+#ifdef CONFIG_ACPI
 static const struct acpi_device_id isl29018_acpi_match[] = {
 	{"ISL29018", isl29018},
 	{"ISL29023", isl29023},
 	{"ISL29035", isl29035},
 	{},
 };
 MODULE_DEVICE_TABLE(acpi, isl29018_acpi_match);
+#endif
 
 static const struct i2c_device_id isl29018_id[] = {
 	{"isl29018", isl29018},