mixer_api: moved functions to mixer_control.c

mixer_control.h should provide the functions needed to manipulate a
mixer, without exposing the internal mixer API (which is provided by
mixer_api.h).

src/output/alsa_plugin.c

@@ -19,6 +19,7 @@
 
 #include "../output_api.h"
 #include "../mixer_api.h"
+#include "mixer_control.h"
 
 #include <glib.h>
 #include <alsa/asoundlib.h>

src/output/oss_plugin.c

@@ -19,6 +19,7 @@
 
 #include "../output_api.h"
 #include "../mixer_api.h"
+#include "mixer_control.h"
 
 #include <glib.h>
 

src/output/pulse_plugin.c

@@ -19,6 +19,7 @@
 
 #include "../output_api.h"
 #include "../mixer_api.h"
+#include "mixer_control.h"
 
 #include <glib.h>
 #include <pulse/simple.h>