[media] stk1160: Build as a module if SND is m and audio support is selected

As reported by Randy Dunlap:
When CONFIG_SND=m and CONFIG_SND_AC97_CODEC=m and
CONFIG_VIDEO_STK1160=y
CONFIG_VIDEO_STK1160_AC97=y
drivers/built-in.o: In function `stk1160_ac97_register':
(.text+0x122706): undefined reference to `snd_card_create'
drivers/built-in.o: In function `stk1160_ac97_register':
(.text+0x1227b2): undefined reference to `snd_ac97_bus'
drivers/built-in.o: In function `stk1160_ac97_register':
(.text+0x1227cd): undefined reference to `snd_card_free'
drivers/built-in.o: In function `stk1160_ac97_register':
(.text+0x12281b): undefined reference to `snd_ac97_mixer'
drivers/built-in.o: In function `stk1160_ac97_register':
(.text+0x122832): undefined reference to `snd_card_register'
drivers/built-in.o: In function `stk1160_ac97_unregister':
(.text+0x12285e): undefined reference to `snd_card_free'

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>