Fix one spot where we should have used OIIO::ofstream instead of std. (#1424)
Fix one spot where we should have used OIIO::ofstream instead of std. (#1424)

This is primarily for the sake of mingw, but also is the proper kind
of stream to pass to OIIO::Filesystem::open.

Signed-off-by: Larry Gritz <f0a1ce6f80112d72f7276457fd261d671b827567@larrygritz.com>
brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,aconty/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,brechtvl/OpenShadingLanguage,brechtvl/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,aconty/OpenShadingLanguage