Make gn check //mojo/public/* pass

mojo/public/cpp/utility/tests/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # GYP version: mojo/mojo_public_tests.gypi:mojo_public_utility_unittests
 test("mojo_public_utility_unittests") {
   deps = [
     "//mojo/common/test:run_all_unittests",
+    "//mojo/public/cpp/system",
     "//mojo/public/cpp/test_support:test_utils",
     "//mojo/public/cpp/utility",
     "//testing/gtest",
   ]
 
   sources = [
     "mutex_unittest.cc",
     "run_loop_unittest.cc",
     "thread_unittest.cc",
   ]
 }