graphics/openusd: fix cmake file

Consumers using cmake failed with this error:

CMake Error at /usr/local/cmake/pxrTargets.cmake:426 (set_target_properties):

The link interface of target "garch" contains:

  OpenGL::GL

but the target was not found.

See https://github.com/PixarAnimationStudios/OpenUSD/issues/3310

PR: 296774
Approved by: maintainer’s time-out