netkitbase: mark as deprecated

It is no longer being developed, the FTP is dead, and Debian has
removed it from the distribution.

Also it allows to not worry about adding RPC support through libtirpc
to this package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>