Add librt to default libs on linux

gpu/khronos_glcts_support/BUILD.gn

 # Copyright 2015 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("//testing/test.gni")
 
 declare_args() {
   internal_khronos_glcts_tests = false
 }
 
@@ skipping 120 matching lines @@
   }
   source_set("deutil") {
     sources = khronos_glcts_gypi.deutil_srcs
 
     deps = [
       ":debase",
       ":depool",
       ":dethread",
     ]
 
-    libs = [ "rt" ]
-
     configs += [ ":defaults_config" ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
     configs -= [ "//build/config/compiler:no_rtti" ]
     configs += [ "//build/config/compiler:rtti" ]
 
     public_configs = [ ":deutil_config" ]
 
     if (is_linux) {
       cflags_c = [ "-Wno-string-conversion" ]
@@ skipping 406 matching lines @@
   ]
 
   if (internal_khronos_glcts_tests) {
     sources += [ "$target_gen_dir/khronos_glcts_test_autogen.cc" ]
     deps += [
       ":generate_khronos_glcts_tests",
       ":khronos_glcts_test_windowless",
     ]
   }
 }