anv: Give the installed intel_icd.json file an absolute path

anv: Give the installed intel_icd.json file an absolute path

Not providing a path allows the ICD to work on multi-arch systems but
breaks it if you install anywhere other than /usr/lib.  Given that users
may be installing locally in .local or similar, we probably do want to
provide a filename.  Distros can carry a revert of this commit if they want
an intel_icd.json file without the path.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Chad Versace <chad@kiwitree.net>