Remove unneeded includes.

src/sound.cpp

@@ -14,6 +14,8 @@
 */
 
 #include "global.hpp"
+
+#include "config.hpp"
 #include "filesystem.hpp"
 #include "foreach.hpp"
 #include "game_preferences.hpp"

src/sound.hpp

@@ -15,11 +15,12 @@
 #ifndef SOUND_HPP_INCLUDED
 #define SOUND_HPP_INCLUDED
 
-#include <string>
-#include <vector>
-#include "config.hpp"
 #include "events.hpp"
 
+#include <string>
+
+class config;
+
 namespace sound {
 
 enum channel_group {