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>