V4L/DVB (4614): Export symbol saa7134_tvaudio_setmute from saa7134 for saa7134-alsa

saa7134-alsa now unmutes/mutes the line when opening/closing the capture
device, and needs this symbol

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>