libmbus: fix upstream location

The libmbus tarball is no longer available at
http://www.freescada.com/public-dist. Apparently, the new upstream is
at http://www.rscada.se/libmbus/ and tarballs are at
http://www.rscada.se/public-dist/

. Without this patch, the current
download of libmbus-0.8.0.tar.gz downloads an HTML document with an
error, which cannot be extracted, obviously.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>