use Mix_Music from SDL_Mixer directly

client/media/CMusicHandler.cpp

@@ -22,8 +22,6 @@
 #include "../../lib/TerrainHandler.h"
 #include "../../lib/filesystem/Filesystem.h"
 
-#include <SDL_mixer.h>
-
 void CMusicHandler::onVolumeChange(const JsonNode & volumeNode)
 {
 	setVolume(volumeNode.Integer());

client/media/CMusicHandler.h

@@ -14,8 +14,7 @@
 
 #include "../lib/CConfigHandler.h"
 
-struct _Mix_Music;
-using Mix_Music = struct _Mix_Music;
+#include <SDL_mixer.h>
 
 class CMusicHandler;