package/iftop: fix static build

Use pcap-config to list optional libpcap dependencies that we need to list
when building statically.

Inspired by Baruch Siach
http://git.buildroot.net/buildroot/commit/package/dhcpdump/dhcpdump.mk?id=429f4415cd153c6809394a8b3245d4d15bba3ec3

Fixes
http://autobuild.buildroot.net/results/c7e/c7e3b2897a9cb9ab55dc7b1a2cd1961235d2d1a4/
http://autobuild.buildroot.net/results/60f/60f82bedae255f6b69c9a5ac22686c76c6276301/
http://autobuild.buildroot.net/results/1b7/1b771af04a95a78144141a5d555c97bbb5a7e13f/
http://autobuild.buildroot.net/results/1f4/1f45e2adcc80c4a209aa5895260985460933575c/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>