Fix OSLTarget.cmake reference path

[luyatshimbalanga]

Due to this change from OSL years ago, it will be better address the possible issue when a packager/maintainer manually relocates /usr/lib64/osl.imageio.so to /usr/lib64/OpenImageIO-3.1/osl.imageio.so leading to the following failure:

-- Setting namespace to 'MaterialX_v1_39_4'
CMake Error at /usr/lib64/cmake/OSL/OSLTargets.cmake:140 (message):
  The imported target "OSL::osl.imageio" references the file

     "/usr/lib64/osl.imageio.so"

  but this file does not exist.  Possible reasons include:

  * The file was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and contained

     "/usr/lib64/cmake/OSL/OSLTargets.cmake"

  but not all the files it references.

Call Stack (most recent call first):
  /usr/lib64/cmake/OSL/OSLConfig.cmake:61 (include)
  CMakeLists.txt:219 (find_package)

where the actual location is /usr/lib64/OpenImageIO-3.1/osl.imageio.so.