usb: gadget: Fix android compilation error

Recent audio composition support in Android gadget driver depends
on CONFIG_SND_PCM. Conditionally include the audio function only
if this is defined.

Change-Id: I0e61687e93e2c4002a3df8618147278f7df6dc7d
Signed-off-by: Jack Pham <jackp@codeaurora.org>