popt: change POPT_SITE back to original url

The popt-1.16.tar.gz file is no more available at current URL and
rmp5.org is online.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>