Add missing header files to GN files

ppapi/native_client/src/untrusted/irt_stub/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.
 
 assert(is_nacl,
        "These targets must only be built using the untrusted NaCl toolchains.")
 
 static_library("ppapi_stub_lib") {
   output_name = "ppapi_stub"
 
   # This library is distributed as a part of the SDK and as such has to
   # be a static library rather than a source set.
   sources = [
     "plugin_main_irt.c",
     "ppapi_plugin_main.c",
     "ppapi_plugin_start.c",
+    "ppapi_start.h",
     "thread_creator.c",
+    "thread_creator.h",
   ]
   configs -= [ "//build/config/gcc:symbol_visibility_hidden" ]
   configs += [ "//build/config/gcc:symbol_visibility_default" ]
   cflags = [ "-fPIC" ]
   deps = [
     "//build/config/nacl:nacl_base",
   ]
 }