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>

drivers/iio/chemical/Kconfig

@@ -71,6 +71,19 @@ config PMS7003
 	  To compile this driver as a module, choose M here: the module will
 	  be called pms7003.
 
+config SENSIRION_SGP30
+	tristate "Sensirion SGPxx gas sensors"
+	depends on I2C
+	select CRC8
+	help
+	  Say Y here to build I2C interface support for the following
+	  Sensirion SGP gas sensors:
+	    * SGP30 gas sensor
+	    * SGPC3 low power gas sensor
+
+	  To compile this driver as module, choose M here: the
+	  module will be called sgp30.
+
 config SPS30
 	tristate "SPS30 particulate matter sensor"
 	depends on I2C