Update OpenCL loader search in CMake

[ndgrigorian]

This PR updates the OpenCL loader search used when building dpctl, as pi_opencl is no longer used, and the open-source compiler is currently not packaging libur_adapter_opencl.so.

By adding OpenCL to the search, a libOpenCL.so installed by ocl-icd can be linked to in place of the UR adapter if it is not found.

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