headers_check fix: linux/sound.h

fix the following 'make headers_check' warnings:

  usr/include/linux/sound.h:33: extern's make no sense in userspace
  usr/include/linux/sound.h:34: extern's make no sense in userspace
  usr/include/linux/sound.h:35: extern's make no sense in userspace
  usr/include/linux/sound.h:36: extern's make no sense in userspace
  usr/include/linux/sound.h:37: extern's make no sense in userspace
  usr/include/linux/sound.h:39: extern's make no sense in userspace
  usr/include/linux/sound.h:40: extern's make no sense in userspace
  usr/include/linux/sound.h:41: extern's make no sense in userspace
  usr/include/linux/sound.h:42: extern's make no sense in userspace

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>