song: converted typedef Song to struct song

Again, a data type which can be forward-declared.

src/decoder_api.c

@@ -22,6 +22,7 @@
 #include "decoder_control.h"
 #include "player_control.h"
 #include "audio.h"
+#include "song.h"
 
 #include "utils.h"
 #include "normalize.h"