message: Restore FlatBufferBuilder::kFileIdentifierLength.
  Restore flatbuffers::FlatBufferBuilder::kFileIdentifierLength, which was a documented part of the public API, but the identifier was lost during the refactoring effected by comment commit
diff: @@ -1075,6 +1075,10 @@ class FlatBufferBuilder {
  buf_.swap_allocator(other.buf_);
  }
  + /// @brief The length of a FlatBuffer file header.
  + static const size_t kFileIdentifierLength =
  + ::flatbuffers::kFileIdentifierLength;
  +
  protected:
  // You shouldn't really be copying instances of this class.
  FlatBufferBuilder(const FlatBufferBuilder &);
  