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>

include/linux/sound.h

@@ -25,6 +25,7 @@
 #define SND_DEV_AMIDI		13	/* Like /dev/midi (obsolete) */
 #define SND_DEV_ADMMIDI		14	/* Like /dev/dmmidi (onsolete) */
 
+#ifdef __KERNEL__
 /*
  *	Sound core interface functions
  */
@@ -40,3 +41,4 @@ extern void unregister_sound_special(int unit);
 extern void unregister_sound_mixer(int unit);
 extern void unregister_sound_midi(int unit);
 extern void unregister_sound_dsp(int unit);
+#endif /* __KERNEL__ */