Add "rt" to libs of GN build linux sandbox unittests.

sandbox/linux/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.
 
 import("//build/config/features.gni")
 
 declare_args() {
   compile_suid_client = is_linux
 
   compile_credentials = is_linux
@@ skipping 68 matching lines @@
   ]
 
   deps = [
     ":sandbox",
     ":sandbox_linux_test_utils",
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 
+  libs = [ "rt" ]
+
   if (compile_suid_client) {
     sources += [
       "suid/client/setuid_sandbox_client_unittest.cc",
     ]
   }
   if (use_seccomp_bpf) {
     sources += [
       "bpf_dsl/bpf_dsl_unittest.cc",
       "bpf_dsl/cons_unittest.cc",
       "seccomp-bpf-helpers/baseline_policy_unittest.cc",
@@ skipping 208 matching lines @@
   #      'type': 'none',
   #      'variables': {
   #        'test_suite_name': 'sandbox_linux_jni_unittests',
   #      },
   #      'dependencies': [
   #        'sandbox_linux_jni_unittests',
   #      ],
   #      'includes': [ '../../build/apk_test.gypi' ],
   #      }
 }