ALSA: usx2y: Move UAPI definition into include/uapi/sound/usb_stream.h

The user-space API definition for usb_stream stuff should be moved
to include/uapi/sound to be exposed publicly.

While we're at it, add the missing ifdef guard for double inclusion,
too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>