patch 9.0.0800: compiler complains about repeated typedef

Problem:    Compiler complains about repeated typedef.
Solution:   Remove one typedef.
diff --git a/src/sound.c b/src/sound.c
index f0f5ab0..17ff78b 100644
--- a/src/sound.c
+++ b/src/sound.c
@@ -17,7 +17,7 @@
 
 static long	    sound_id = 0;
 
-typedef struct soundcb_S soundcb_T;
+// soundcb_T is typdef'ed in proto/sound.pro
 
 struct soundcb_S {
     callback_T	snd_callback;