add opencl-headers to meta.yaml

[ndgrigorian]

This PR applies a work-around for a missing dependency on opencl-headers in the intel-sycl-rt package, which removed the headers in fixing clobbering when installed from Intel channel alongside ocl-icd and ocl-icd-system

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[coveralls]

coverage: 74.579%. remained the same — workaround-ocl-headers-windows into master