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_command.c

@@ -28,7 +28,7 @@
 #include "output_all.h"
 #include "output_internal.h"
 #include "output_plugin.h"
-#include "mixer_api.h"
+#include "mixer_control.h"
 #include "idle.h"
 
 bool