Add missing copyright statement to ui_strings.gyp.

BUG=None
TEST=None
TBR=ben@chromium.org

Review URL: https://codereview.chromium.org/83513006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236838 0039d316-1c4b-4281-b951-d872f2087c98

ui/base/strings/ui_strings.gyp

+# Copyright 2013 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.
+
 {
   'variables': {
     'chromium_code': 1,