netsniff-ng: fix static build with netfilter_conntrack

netsniff-ng: fix static build with netfilter_conntrack

Use pkg-config to determine link flags.

Fixes:
http://autobuild.buildroot.net/results/0ef/0efb90d69fdffafe0630e6507d40198bfb1c5818/
http://autobuild.buildroot.net/results/0ea/0eaf15dd387961641b42d977c49804fe8872074b/
http://autobuild.buildroot.net/results/43d/43d3f2d54deb9d095a4f8cbf70a7b933c09ed618/

Cc: Joris Lijssens <joris.lijssens@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>