Use the common location for libnet include files and libraries.

Signed-off-by: Stefan Tomanek <stefan.tomanek@wertarbyte.de>

The most recent developer of netdiscover had self-hosted downloads
but also put the project on SourceForge.  The self-hosted site went
down but SourceForge is much more reliable so I have moved the
download URI to SourceForge (md5sum is the same).

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>

Signed-off-by: Marko Ratkaj <marko.ratkaj@sartura.hr>

Signed-off-by: Mislav Novakovic <mislav.novakovic@sartura.hr>