[media] radio-si476x: depend on SND_SOC

It is not possible to select SND_SOC_SI476X if we have not also
enabled SND_SOC.
warning: (RADIO_SI476X) selects SND_SOC_SI476X which has unmet
	 direct dependencies (SOUND && !M68K && !UML && SND && SND_SOC)

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[hans.verkuil@cisco.com: fixed wrong driver name in subject]
Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>