Add debuginfod_url to dist macros
    
    elfutils has a per-distribution URL pointing to a debuginfod
    server.  Add this field to dist macros.  It's not exposed via
    /etc/os-release as it doesn't belong there.
    
    (cherry picked from commit 3fed407628726f97a94d6051d908914ec92c9385)
    Signed-off-by: Amit Shah <amitshah@fedoraproject.org>