lp8788-charger: Fix kconfig dependency

Fix build errors in lp8788-charger by making it depend on IIO.
Fixes errors when CONFIG_IIO=m and CHARGER_LP8788=y.

lp8788-charger.c:(.text+0x2146b5): undefined reference to `iio_channel_get'
lp8788-charger.c:(.text+0x2146ce): undefined reference to `iio_channel_get'
lp8788-charger.c:(.text+0x214a86): undefined reference to `iio_read_channel_processed'
lp8788-charger.c:(.text+0x214b51): undefined reference to `iio_read_channel_processed'
lp8788-charger.c:(.text+0x214c30): undefined reference to `iio_read_channel_processed'
lp8788-charger.c:(.text+0x214d93): undefined reference to `iio_channel_release'
lp8788-charger.c:(.text+0x214dac): undefined reference to `iio_channel_release'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>