ui: Add libsync GLFence implementation.

third_party/libsync/BUILD.gn

 # Copyright 2016 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.
 
 config("libsync_config") {
   include_dirs = [ "include" ]
 }
 
 source_set("libsync") {
   sources = [
     "include/linux/sw_sync.h",
     "include/linux/sync.h",
     "include/sw_sync.h",
     "include/sync/sync.h",
     "strlcpy.c",
     "sync.c",
   ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 
   public_configs = [ ":libsync_config" ]
+
+  cflags_c = [ "-Wno-implicit-function-declaration" ]
 }