service/audio: Replace includes with forward declarations where applicable

A few headers were including other headers when a forward declaration
can be used instead, allowing the include to be moved to the cpp file.

src/core/hle/service/audio/hwopus.cpp

@@ -3,7 +3,12 @@
 // Refer to the license.txt file included.
 
 #include <cstring>
+#include <memory>
+#include <vector>
+
 #include <opus.h>
+
+#include "common/common_funcs.h"
 #include "common/logging/log.h"
 #include "core/hle/ipc_helpers.h"
 #include "core/hle/kernel/hle_ipc.h"