[fuchsia] Move async_get_default_dispatcher include to the header (flutter#14351)

Author: iskakaushik

shell/platform/fuchsia/flutter/thread.cc

@@ -10,7 +10,6 @@
 #include <algorithm>
 
 #include <lib/async-loop/cpp/loop.h>
-#include <lib/async/default.h>
 
 #include "flutter/fml/logging.h"
 #include "loop.h"

shell/platform/fuchsia/flutter/thread.h

@@ -10,6 +10,7 @@
 #include <functional>
 
 #include <lib/async-loop/cpp/loop.h>
+#include <lib/async/default.h>
 
 #include "flutter/fml/macros.h"