mesa3d: fix download URL

The MesaLib 7.10.3 tarball has been moved on ftp.freedesktop.org from
pub/mesa/7.10.3/ to pub/mesa/old_versions/7.x/7.10.3/.
A bump to a newer release is only coming after Buildroot 2013.08, so for now
let us just change URL to let the download work properly.

Reported-by: Ryan Wilkins <ryan@deadfrog.net>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>