media: cx231xx: Add I2C_MUX dependency

cx231xx requires i2c mux adapter capability.
Signed-off-by: NBrad Love <brad@nextdimension.cc>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/usb/cx231xx/Kconfig

 config VIDEO_CX231XX
 	tristate "Conexant cx231xx USB video capture support"
-	depends on VIDEO_DEV && I2C
+	depends on VIDEO_DEV && I2C && I2C_MUX
 	select VIDEO_TUNER
 	select VIDEO_TVEEPROM
 	select VIDEOBUF_VMALLOC