knock: fix static build

Use pcap-config to determine link dependencies of libpcap.

Fixes:
http://autobuild.buildroot.net/results/1eb/1eb877ebd9469c9261bf114fc7733819ae3fd562/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>