drivers/media/video/em28xx/: Fix undefined symbol error with CONFIG_SND=N

Without this you get undefined symbol errors with CONFIG_SND=N:

  ERROR: "snd_pcm_period_elapsed" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_pcm_hw_constraint_integer" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_pcm_set_ops" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_pcm_lib_ioctl" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_card_new" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_card_free" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_card_register" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!
  ERROR: "snd_pcm_new" [drivers/media/video/em28xx/em28xx-alsa.ko] undefined!

Signed-off-by: S.Çağlar Onur <caglar@pardus.org.tr>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>