V4L/DVB (6794): Fix compilation when dib3000mc is compiled as a module

As reported by Andrew Morton:

> ERROR: "dibusb_dib3000mc_frontend_attach" [drivers/media/dvb/dvb-usb/dvb-usb-nova-t-usb2.ko] undefined!
> ERROR: "dibusb_dib3000mc_tuner_attach" [drivers/media/dvb/dvb-usb/dvb-usb-nova-t-usb2.ko] undefined!
> ERROR: "dibusb_dib3000mc_frontend_attach" [drivers/media/dvb/dvb-usb/dvb-usb-dibusb-mc.ko] undefined!
> ERROR: "dibusb_dib3000mc_tuner_attach" [drivers/media/dvb/dvb-usb/dvb-usb-dibusb-mc.ko] undefined!
> ERROR: "dibusb_dib3000mc_frontend_attach" [drivers/media/dvb/dvb-usb/dvb-usb-a800.ko] undefined!
> ERROR: "dibusb_dib3000mc_tuner_attach" [drivers/media/dvb/dvb-usb/dvb-usb-a800.ko] undefined

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>