include SDL_mixer.h directly in CMusicHandler.h where it's used

Index: client/CMusicHandler.h
--- client/CMusicHandler.h.orig
+++ client/CMusicHandler.h
@@ -9,12 +9,12 @@
  */
 #pragma once
 
+#include <SDL_mixer.h>
+
 #include "../lib/CConfigHandler.h"
 #include "../lib/CSoundBase.h"
 
-struct _Mix_Music;
 struct SDL_RWops;
-using Mix_Music = struct _Mix_Music;
 struct Mix_Chunk;
 
 class CAudioBase {
