Add missing header files to GN files

components/nacl/renderer/plugin/BUILD.gn

 # Copyright (c) 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.
 
 static_library("nacl_trusted_plugin") {
   sources = [
     "module_ppapi.cc",
     "module_ppapi.h",
     "nacl_subprocess.cc",
     "nacl_subprocess.h",
     "plugin.cc",
+    "plugin_error.h",
     "pnacl_coordinator.cc",
     "pnacl_coordinator.h",
     "pnacl_resources.cc",
     "pnacl_resources.h",
     "pnacl_translate_thread.cc",
     "pnacl_translate_thread.h",
     "ppapi_entrypoints.cc",
     "ppapi_entrypoints.h",
     "service_runtime.cc",
     "service_runtime.h",
   ]
 
   deps = [
     "//base",
     "//components/nacl/common:nacl_error_code",
     "//content/public/common",
     "//media:shared_memory_support",
     "//ppapi/c",
     "//ppapi/cpp:objects",
     "//ppapi/cpp/private:internal_module",
     "//ppapi/proxy:ipc",
   ]
 }