default constructor must be declared first.

Index: src/mixer.h
--- src/mixer.h.orig
+++ src/mixer.h
@@ -46,7 +46,7 @@ class Mixer (private)
     bool audioHWInitialized;
 
 public:
-    Mixer(MixerMode mmode);
+    Mixer(MixerMode mmode = Realtime);
     ~Mixer();
     void Play();
     void Pause();
