bustle: fix static build

Use pcap-config to list optional libpcap dependencies needed for static link.

Fixes:
http://autobuild.buildroot.net/results/569/56987b9c39376d2443b96160485f4fd8eefda9bb/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>