[media] usbtv: Make it dependent on ALSA

[media] usbtv: Make it dependent on ALSA

Now that alsa code is part of the driver, it can be compiled
only if alsa is enabled.

   drivers/built-in.o: In function `snd_usbtv_hw_free':
>> usbtv-audio.c:(.text+0x21eb55): undefined reference to `snd_pcm_lib_free_pages'
   drivers/built-in.o: In function `snd_usbtv_hw_params':
>> usbtv-audio.c:(.text+0x21eb72): undefined reference to `snd_pcm_lib_malloc_pages'
   drivers/built-in.o: In function `usbtv_audio_urb_received':
>> usbtv-audio.c:(.text+0x21ed66): undefined reference to `snd_pcm_link_rwlock'
>> usbtv-audio.c:(.text+0x21ed9f): undefined reference to `snd_pcm_link_rwlock'
>> usbtv-audio.c:(.text+0x21edf5): undefined reference to `snd_pcm_period_elapsed'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f00a): undefined reference to `snd_card_new'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f0a2): undefined reference to `snd_pcm_new'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f0e5): undefined reference to `snd_pcm_set_ops'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f103): undefined reference to `snd_pcm_lib_preallocate_pages_for_all'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f10c): undefined reference to `snd_card_register'
   drivers/built-in.o: In function `usbtv_audio_init':
>> (.text+0x21f12a): undefined reference to `snd_card_free'
   drivers/built-in.o: In function `usbtv_audio_free':
>> (.text+0x21f15c): undefined reference to `snd_card_free'
>> drivers/built-in.o:(.data+0x43250): undefined reference to `snd_pcm_lib_ioctl'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>