decoder_list: convert to C++

test/dump_playlist.cxx

@@ -24,16 +24,13 @@
 #include "input_stream.h"
 #include "conf.h"
 #include "decoder_api.h"
+#include "DecoderList.hxx"
 #include "InputInit.hxx"
 #include "IOThread.hxx"
 #include "PlaylistRegistry.hxx"
 #include "PlaylistPlugin.hxx"
 #include "fs/Path.hxx"
 
-extern "C" {
-#include "decoder_list.h"
-}
-
 #include <glib.h>
 
 #include <unistd.h>