tcpreplay: use pcap-config

Since other libs can be pulled in by libpcap switch to using
pcap-config, it's the correct way and simpler. Fixes:
http://autobuild.buildroot.net/results/c02/c023bacd35e9ef1addeb881cd3d572f97f9b4b56/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>