Unbreak build with Boost 1.69

[jbeich]

After boostorg/serialization@af46ac2da3e9 (post-Beta1) build fails. See error log.

src/systems/base/gan_graphics_object_data.cc:56:12: error: no member named 'cerr' in namespace 'std'
using std::cerr;
      ~~~~~^

[eglaysher]

Thank you!