Blender Git Loki

CMake/OpenSubdiv: Rename INCLUDE_DIR -> INCLUDE_DIRS.

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