Graph editor fails to compile with OCIO enabled

[sambler]

When I build with graph editor and ocio enabled, I get a compile error as mx::OcioColorManagementSystem doesn't exist.

I fixed the build by choosing between ColorManagement headers

#ifdef MATERIALX_BUILD_OCIO
#include <MaterialXGenShader/OcioColorManagementSystem.h>
#else
#include <MaterialXGenShader/DefaultColorManagementSystem.h>
#endif

[jstone-lucasfilm]

Thanks for this report, @sambler, and this should now be addressed in #2780.