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 <[e-mail]>
Acked-by: David Brownell <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/Kconfig

···
 config USB_AUDIO
 	tristate "Audio Gadget (EXPERIMENTAL)"
 	depends on SND
+	select SND_PCM
 	help
 	  Gadget Audio is compatible with USB Audio Class specification 1.0.
 	  It will include at least one AudioControl interface, zero or more