include/linux/usb/audio.h: add __attribute__((packed))

This was missing on the definition of struct uac_iso_endpoint_descriptor

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>