mixer_api: moved mixer_plugin imports to mixer_list.h

This patch allows the output plugins to import only mixer_list.h,
instead of the full mixer_api.h (which would expose internal
structures).

src/output/alsa_plugin.c

@@ -18,7 +18,7 @@
  */
 
 #include "../output_api.h"
-#include "../mixer_api.h"
+#include "mixer_list.h"
 #include "mixer_control.h"
 
 #include <glib.h>

src/output/oss_plugin.c

@@ -18,7 +18,7 @@
  */
 
 #include "../output_api.h"
-#include "../mixer_api.h"
+#include "mixer_list.h"
 #include "mixer_control.h"
 
 #include <glib.h>

src/output/pulse_plugin.c

@@ -18,7 +18,7 @@
  */
 
 #include "../output_api.h"
-#include "../mixer_api.h"
+#include "mixer_list.h"
 #include "mixer_control.h"
 
 #include <glib.h>