snmppp: update download URL
The filename of the SNMP++ sources has changed.
The old one is still working because the server automatically redirects to the
new file, but it is safer to use the official name.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Luca Ceresoli