In the past, we modified contrib/libpcap/pcap/pcap.h to include
net/bpf.h rather than libpcap's own bpf.h. Revert that change, because
libpcap's cut-down version of bpf.h only includes components necessary
for the code generator, userland BPF interpreter, and the libpcap APIs
for setting filters, etc.. pcap-bpf.c includes the native OS version.
This is in preparation for a libpcap update to 1.10.3 where pcap/pcap.h
includes headers within the pcap source tree. Continuing to directly
include our net/bpf.h would require more local changes.