audio_manager: Forward declare result type

Moves the include into the cpp file to lessen header dependencies.

src/audio_core/audio_manager.cpp

@@ -3,6 +3,7 @@
 
 #include "audio_core/audio_manager.h"
 #include "core/core.h"
+#include "core/hle/service/audio/errors.h"
 
 namespace AudioCore {
 

src/audio_core/audio_manager.h

@@ -10,7 +10,8 @@
 #include <thread>
 
 #include "audio_core/audio_event.h"
-#include "core/hle/service/audio/errors.h"
+
+union Result;
 
 namespace AudioCore {