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>