Fix build break against OpenColorIO master -- headers disappeared (#2817)
Fix build break against OpenColorIO master -- headers disappeared (#2817)

The functions we depend on are all still there, they just moved from
these headers into the main OpenColorIO.h.
lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio