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

Index: client/CMusicHandler.cpp
--- client/CMusicHandler.cpp.orig
+++ client/CMusicHandler.cpp
@@ -8,7 +8,6 @@
  *
  */
 #include "StdInc.h"
-#include <SDL_mixer.h>
 #include <SDL_timer.h>
 
 #include "CMusicHandler.h"
