Audio:mixin_mixout: move mixin_mixouot header file to module folder

This is a clean up, purpose is de-cluster headers, toml files,
Readme.md etc per module basis, since today everything is scattered
in current code base.

Signed-off-by: Baofeng Tian <baofeng.tian@intel.com>

src/audio/mixin_mixout/mixin_mixout.c

@@ -26,11 +26,12 @@
 #include <ipc/stream.h>
 #include <ipc/topology.h>
 #include <ipc4/base-config.h>
-#include <ipc4/mixin_mixout.h>
 #include <user/trace.h>
 #include <stddef.h>
 #include <stdint.h>
 
+#include "mixin_mixout.h"
+
 LOG_MODULE_REGISTER(mixer, CONFIG_SOF_LOG_LEVEL);
 
 /* mixin 39656eb2-3b71-4049-8d3f-f92cd5c43c09 */

src/audio/mixin_mixout/mixin_mixout_generic.c

@@ -4,10 +4,11 @@
 //
 // Author: Andrula Song <xiaoyuan.song@intel.com>
 
-#include <ipc4/mixin_mixout.h>
 #include <sof/common.h>
 #include <rtos/string.h>
 
+#include "mixin_mixout.h"
+
 #ifdef MIXIN_MIXOUT_GENERIC
 
 #if CONFIG_FORMAT_S16LE

src/audio/mixin_mixout/mixin_mixout_hifi3.c

@@ -4,9 +4,10 @@
 //
 // Author: Andrula Song <xiaoyuan.song@intel.com>
 
-#include <ipc4/mixin_mixout.h>
 #include <sof/common.h>
 
+#include "mixin_mixout.h"
+
 #ifdef MIXIN_MIXOUT_HIFI3
 
 #if CONFIG_FORMAT_S16LE