stream: Replace includes with forward declarations where applicable

Avoids propagating includes in headers where it's not necessary to do
so.

src/audio_core/stream.cpp

@@ -7,6 +7,7 @@
 
 #include "audio_core/sink.h"
 #include "audio_core/sink_details.h"
+#include "audio_core/sink_stream.h"
 #include "audio_core/stream.h"
 #include "common/assert.h"
 #include "common/logging/log.h"

src/audio_core/stream.h

@@ -11,13 +11,16 @@
 #include <queue>
 
 #include "audio_core/buffer.h"
-#include "audio_core/sink_stream.h"
-#include "common/assert.h"
 #include "common/common_types.h"
-#include "core/core_timing.h"
+
+namespace CoreTiming {
+struct EventType;
+}
 
 namespace AudioCore {
 
+class SinkStream;
+
 /**
  * Represents an audio stream, which is a sequence of queued buffers, to be outputed by AudioOut
  */