graphics/py-openimageio: unbreak with boost 1.67

• Boost python support not found -- will not build python components!

[...]
install -s -m 0644 /wrkdirs/usr/ports/graphics/py-openimageio/work/.build/src/python/OpenImageIO.so /wrkdirs/usr/ports/graphics/py-openimageio/work/stage/usr/local/lib/python2.7/site-packages
install: /wrkdirs/usr/ports/graphics/py-openimageio/work/.build/src/python/OpenImageIO.so: No such file or directory

PR: 227427
Reported by: antoine (via exp-run)