package/mxml: fix download URL

The project moved to github, the current download URL is broken:

$ wget -q  http://www.msweet.org/files/project3/mxml-2.10.tar.gz


$ file mxml-2.10.tar.gz
mxml-2.10.tar.gz: HTML document, UTF-8 Unicode text, with very long lines

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>