Fix check for level zero headers

[ndgrigorian]

This PR updates the GetLevelZeroHeaders.cmake

The paths defined previously in parts of the CMake file did not work and were inconsistent, using CMAKE_BINARY_DIR in some places (which worked) and relative paths in others

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[github-actions]

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

[coveralls]

coverage: 74.462%. remained the same — update-getlevelzeroheaders into master

[antonwolfy]

LGTM