Add header guards

src/sound.h

@@ -18,6 +18,9 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
  */
 
+#ifndef _FCEU_SOUND_H
+#define _FCEU_SOUND_H
+
 typedef struct {
 	void (*Fill)(int Count);	/* Low quality ext sound. */
 
@@ -60,3 +63,5 @@ void FCEUSND_SaveState(void);
 void FCEUSND_LoadState(int version);
 
 void FASTAPASS(1) FCEU_SoundCPUHook(int);
+
+#endif