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: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>