[media] cx231xx: Fix compilation breakage if DVB is not selected

In file included from drivers/media/video/cx231xx/cx231xx-audio.c:40:
drivers/media/video/cx231xx/cx231xx.h:559: error: field ‘frontends’ has incomplete type
make[4]: ** [drivers/media/video/cx231xx/cx231xx-audio.o] Erro 1
make[3]: ** [drivers/media/video/cx231xx] Erro 2
make[2]: ** [drivers/media/video] Erro 2
make[1]: ** [drivers/media] Erro 2
make: ** [drivers] Erro 2

Reported-by: Marcio Araujo Alves <froooozen@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>