In addition to the pcap lib in /usr/lib, you also need the header files in /usr/include/pcap. Do you have a directory /usr/include pcap? If you do, then one can only assume you have a mismatch between your header files and the actual library. Maybe you have 2 versions installed, another in /usr/local/lib possibly? It would be a wise step to build pcap yourself, from tcpdump and pcap instead of relying on Ubuntu's split packages.