Ref {D8855}
Unix and Apple platform files use find_package(OpenSubdiv) which sets
`OPENSUBDIV_INCLUDE_DIR` as an advanced variable, as well as
`OPENSUBDIV_INCLUDE_DIRS` which should be used usually.
Windows sets `OPENSUBDIV_INCLUDE_DIR` which is used by the rest
of the code.
This patch renames it to `_DIRS` everywhere, for it to be like other
similar variables.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D8917