staging/easycap: fix build when SND is not enabled

Fix easycap build when CONFIG_SOUND is enabled but CONFIG_SND is
not enabled.

use choice construct to select between ALSA and OSS API binding

drivers/built-in.o: In function `easycap_usb_disconnect':
easycap_main.c:(.text+0x2aba20): undefined reference to `snd_card_free'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b784b): undefined reference to `snd_card_create'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b78fb): undefined reference to `snd_pcm_new'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b7916): undefined reference to `snd_pcm_set_ops'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b795b): undefined reference to `snd_card_register'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b79d8): undefined reference to `snd_card_free'
drivers/built-in.o: In function `easycap_alsa_probe':
(.text+0x2b7a78): undefined reference to `snd_card_free'
drivers/built-in.o: In function `easycap_alsa_complete':
(.text+0x2b7e68): undefined reference to `snd_pcm_period_elapsed'
drivers/built-in.o:(.data+0x2cae8): undefined reference to `snd_pcm_lib_ioctl'

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: R.M. Thomas <rmthomas@sciolus.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>