graphics/opensubdiv: fix cmake config file

Consumers encountered this error:

CMake Error at /usr/local/lib/cmake/OpenSubdiv/OpenSubdivTargets.cmake:79 (set_target_properties):

The link interface of target "OpenSubdiv::osdCPU" contains:
  OpenMP::OpenMP_CXX
but the target was not found.

PR: 296775
Approved by: maintainer’s time-out