popt: fix SRC_URI

rpm5.org has been down for about a year now. Use linuxfromscratch.org
as an alternative reliable source instead.

Signed-off-by: Oleksandr Kravchuk <open.source@oleksandr-kravchuk.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d9224014)
Signed-off-by: Armin Kuster <akuster808@gmail.com>