media: dvb-usb-v2: fix a missing dependency of I2C_MUX

Now that af9015 requires I2C_MUX, all drivers that select
it should also depend on it.

   drivers/media/dvb-frontends/af9013.o: In function `af9013_remove':
>> drivers/media/dvb-frontends/af9013.c:1560: undefined reference to `i2c_mux_del_adapters'
   drivers/media/dvb-frontends/af9013.o: In function `af9013_probe':
>> drivers/media/dvb-frontends/af9013.c:1488: undefined reference to `i2c_mux_alloc'
>> drivers/media/dvb-frontends/af9013.c:1495: undefined reference to `i2c_mux_add_adapter'
   drivers/media/dvb-frontends/af9013.c:1544: undefined reference to `i2c_mux_del_adapters'
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/usb/dvb-usb-v2/Kconfig

@@ -15,7 +15,7 @@ config DVB_USB_V2
 
 config DVB_USB_AF9015
 	tristate "Afatech AF9015 DVB-T USB2.0 support"
-	depends on DVB_USB_V2
+	depends on DVB_USB_V2 && I2C_MUX
 	select REGMAP
 	select DVB_AF9013
 	select DVB_PLL              if MEDIA_SUBDRV_AUTOSELECT