tcpdump: fix static build

Commit 746116d1 (tcpdump: use libpcap shared library) broke static build
of tcpdump, because its configure script doesn't take into account indirect
dependencies of libpcap. Add these dependencies to the LIBS configure
parameter.

Fixes:
http://autobuild.buildroot.net/results/fd231d34e4bb0306609b021e9e74eb862b7bd6bd/



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