iio: chemical: fix missing Kconfig block for sgp30

I clearly messed up applying this patch. Not sure
how but the entire Kconfig block is missing. This
patch puts it back as it was in the original patch.

Reported-by: Andreas Brauchli <a.brauchli@elementarea.net>
Fixes: ce514124 ("iio: chemical: sgp30: Support Sensirion SGP30/SGPC3 sensors")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>