dhcpdump: fix static build

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

Fixes:
http://autobuild.buildroot.net/results/110/1107c21cdf656763bf7048c6c5c7899369724f5f/



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