dpctl needs to provide get_include()

[oleksandr-pavlyk]

When building Cython native application that builds on dpctl, include_dir option of the Extension need to point to dpctl/backend/include to pick up dpctl utilities.

dpctl must provide a function that returns the location of that folder in the installation, very much like what numpy.get_include() does. See: https://numpy.org/devdocs/reference/generated/numpy.get_include.html

[oleksandr-pavlyk]

This also means that these header files must be in the installation layout.

[diptorupd]

@oleksandr-pavlyk Since #69 is merged, can this be closed?

[oleksandr-pavlyk]

Yes, this is implemented