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>