song: converted typedef Song to struct song

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

src/decoder_thread.c

@@ -21,6 +21,7 @@
 #include "decoder_control.h"
 #include "decoder_internal.h"
 #include "player_control.h"
+#include "song.h"
 
 #include "path.h"
 #include "log.h"