Add missing .h files to .gn files that have a .cc file next to it in the gn file already.

mojo/edk/system/ports/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.
 
 import("//testing/test.gni")
 
 source_set("ports") {
   sources = [
     "event.cc",
     "event.h",
     "message.cc",
     "message.h",
     "message_filter.h",
     "message_queue.cc",
     "message_queue.h",
     "name.cc",
     "name.h",
     "node.cc",
     "node.h",
     "node_delegate.h",
     "port.cc",
     "port.h",
     "port_ref.cc",
+    "port_ref.h",
     "user_data.h",
   ]
 
   public_deps = [
     "//base",
   ]
 }
 
 source_set("tests") {
   testonly = true
 
   sources = [
     "ports_unittest.cc",
   ]
 
   deps = [
     ":ports",
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }