USB: gadget audio: select SND_PCM

Fix USB gadget audio: select SND_PCM, like many other sound
drivers do, to fix build errors:

drivers/built-in.o: In function `f_audio_playback_work':
audio.c:(.text+0x15a3e7): undefined reference to `snd_pcm_kernel_ioctl'
audio.c:(.text+0x15a471): undefined reference to `snd_pcm_lib_write'
drivers/built-in.o: In function `_snd_pcm_hw_param_set':
audio.c:(.text+0x15aca7): undefined reference to `snd_interval_refine'
drivers/built-in.o: In function `gaudio_setup':
(.init.text+0x12adf): undefined reference to `_snd_pcm_hw_params_any'
drivers/built-in.o: In function `gaudio_setup':
(.init.text+0x12b43): undefined reference to `snd_pcm_kernel_ioctl'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>