message: Fix tpu build.
diff: @@ -14,6 +14,8 @@ limitations under the License.
  ==============================================================================*/
  #include "tensorflow/core/tpu/kernels/tpu_compile_op_support.h"
  +#include <string>
  +
  #include "tensorflow/compiler/xla/debug_options_flags.h"
  #include "tensorflow/compiler/xla/service/computation_layout.h"
  #include "tensorflow/compiler/xla/service/computation_placer.h"
  @@ -345,7 +347,8 @@ StatusOr<TpuCompilationRequestProto> CreateTpuCompilationRequest(
  if (use_mlir) {
  VLOG(1) << "Serializing MlirModule";
  const MlirToHloArgs& mlir_computation = absl::get<0>(computation);
  - *compilation_request.mutable_mlir_module() = mlir_computation.mlir_module;
  + *compilation_request.mutable_mlir_module() =
  + string(mlir_computation.mlir_module);
  } else {
  VLOG(1) << "Serializing FunctionDefinitionLibrary";
  const FunctionToHloArgs& function_computation = absl::get<1>(computation);
  