Fix broken fuchsia test

Author: arbreng

shell/platform/fuchsia/flutter/platform_view_unittest.cc

@@ -7,6 +7,7 @@
 #include <gtest/gtest.h>
 #include <lib/async-loop/cpp/loop.h>
 #include <lib/async-loop/default.h>
+#include <lib/async/default.h>
 #include <lib/fidl/cpp/binding_set.h>
 #include <lib/fidl/cpp/interface_request.h>
 #include <lib/sys/cpp/testing/service_directory_provider.h>