[shell][fuchsia] Add lib/async/default.h

For async_get_default_dispatcher().

Author: cdotstout

shell/platform/fuchsia/flutter/component.cc

@@ -8,6 +8,7 @@
 #include <fuchsia/mem/cpp/fidl.h>
 #include <lib/async-loop/cpp/loop.h>
 #include <lib/async/cpp/task.h>
+#include <lib/async/default.h>
 #include <lib/fdio/directory.h>
 #include <lib/fdio/namespace.h>
 #include <lib/ui/scenic/cpp/view_token_pair.h>