message: add the missing license header for one file
diff: +# Copyright (c) Facebook, Inc. and its affiliates.
  +#
  +# This source code is licensed under the MIT license found in the
  +# LICENSE file in the root directory of this source tree.
  +
  include(ExternalProject)
  ExternalProject_Add(googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  