song: converted typedef Song to struct song

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

src/player_thread.c

@@ -25,6 +25,7 @@
 #include "log.h"
 #include "main_notify.h"
 #include "crossfade.h"
+#include "song.h"
 
 enum xfade_state {
 	XFADE_DISABLED = -1,