[media] rtl28xxu: depends on I2C_MUX

We need depend on I2C_MUX as rtl2832 demod used requires it.

All error/warnings:
warning: (DVB_USB_RTL28XXU) selects DVB_RTL2832 which has unmet direct dependencies (MEDIA_SUPPORT && DVB_CORE && I2C && I2C_MUX)
ERROR: "i2c_add_mux_adapter" [drivers/media/dvb-frontends/rtl2832.ko] undefined!
ERROR: "i2c_del_mux_adapter" [drivers/media/dvb-frontends/rtl2832.ko] undefined!

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>