iio: light: al3320a: add missing REGMAP_I2C to Kconfig

The Kconfig entry for the al3320a is missing a `select REGMAP_I2C`,
causing build failures.

Fixes: 1850e6ae7f91 ("iio: light: al3320a: Implement regmap support")
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: David Heidelberg <david@ixit.cz>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>