message: Fixed poplar BUILD file
  Test Plan: Started a build locally with both bazel 0.29.1 and 0.24.1
  Reviewers: #tensorflow, georgep
diff: @@ -10,11 +10,11 @@ load("@local_config_poplar//poplar:build_defs.bzl", "tf_poplar_build_tag")
  load("//tensorflow:tensorflow.bzl", "tf_cc_binary")
  load("//tensorflow:tensorflow.bzl", "tf_gen_op_wrapper_py")
  load("//tensorflow:tensorflow.bzl", "tf_custom_op_py_library")
  -load("//tensorflow/core:platform/default/build_config.bzl", "tf_proto_library")
  +load("//tensorflow/core/platform:default/build_config.bzl", "tf_proto_library")
  load("//tensorflow/compiler/xla:xla.bzl", "xla_proto_library")
  load("//tensorflow:tensorflow.bzl", "tf_custom_op_library")
  load(
  - "//tensorflow/core:platform/default/build_config.bzl",
  + "//tensorflow/core/platform:default/build_config.bzl",
  "tf_proto_library_py",
  )
  