xl2tp: fix static build

Use pcap-config to list dependencies of libpcap.

Fixes:
http://autobuild.buildroot.net/results/87f/87ff92831d265d437aa0dfcfb99e954f76597761/
http://autobuild.buildroot.net/results/cd5/cd5487e76259752cbe58b171f9a58970916f7bb8/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>