twl4030_charger: add missing iio dependency

This driver fails to link without CONFIG_IIO, since
there are no stubs for the iio_channels functions.

Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Marek Belisko <marek@goldelico.com>
Acked-by: Nikolaus Schaller <hns@goldelico.com>