mixer: include cleanup

Don't include conf.h in mixer_api.h.  Use a forward struct declaration
instead.

src/audio.c

@@ -23,6 +23,7 @@
 #include "output_plugin.h"
 #include "output_all.h"
 #include "mixer_api.h"
+#include "conf.h"
 
 #include <glib.h>
 

src/mixer_api.h

@@ -19,7 +19,7 @@
 #ifndef MPD_MIXER_H
 #define MPD_MIXER_H
 
-#include "conf.h"
+#include <stdbool.h>
 
 /*
  * list of currently implemented mixers
@@ -28,6 +28,8 @@
 extern const struct mixer_plugin alsa_mixer;
 extern const struct mixer_plugin oss_mixer;
 
+struct config_param;
+
 struct mixer_plugin {
 	/**
          * Alocates and configures a mixer device.