Blender Git Loki

libs/windows: Prevent USD exports from leaking into blender binary.

Even though we build USD as static, it still feels the need to mark its
symbols with declspec(dllexport) which means the blender binary now exports
these symbols.

this patch fixes that unwanted behaviour, however USD libs still need to
rebuild before this becomes visible in the blender binary

Differential Revision: https://developer.blender.org/D6563

Reviewed By: sybren