gnu: xl2tpd: Add pppd support.

* gnu/packages/vpn.scm (xl2tpd)[inputs]: Add "linux-libre-headers" and "ppp",
[arguments]: add a 'setup-environment phase to set "PPPD" and "KERNELSRC"
variables.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>