iptables: fix libpcap static linking

libpcap can depend on other libraries like libusb and doesn't support
pkgconfig, so add those libraries to LIBS when building statically.
Fixes:
http://autobuild.buildroot.net/results/b4a/b4a3d00e9673a7aacc663c81de1d8e887a17951d/



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