message: only include tensorflow/stream_executor/platform.h if needed
diff: #include "tensorflow/core/lib/core/status.h"
  #include "tensorflow/core/platform/env.h"
  #include "tensorflow/core/public/session.h"
  -#include "tensorflow/stream_executor/platform.h"
  #if MINIGO_ENABLE_GPU
  #include "tensorflow/core/common_runtime/gpu/gpu_init.h"
  +#include "tensorflow/stream_executor/platform.h"
  #endif
  using tensorflow::DT_FLOAT;
  