default constructor must be declared first.

Index: src/mixer.cpp
--- src/mixer.cpp.orig
+++ src/mixer.cpp
@@ -23,7 +23,7 @@
 
 namespace Scenes
 {
-Mixer::Mixer(MixerMode mmode = Realtime)
+Mixer::Mixer(MixerMode mmode)
         : mode(mmode), isPlaying(false)
 {
     mut_disposeList = SDL_CreateMutex();
