rebar.config.script: change path for pcap header

While the man page for pcap suggests including pcap/pcap.h, some OS'es
(OpenBSD) rely on pcap.h.

/usr/include/pcap.h has this comment which suggests it can be relied
upon to be present:

    For backwards compatibility.

    Note to OS vendors: do NOT get rid of this file!  Many applications
    expect to be able to include <pcap.h>, and at least some of them go
    through contortions in their configure scripts to try to detect OSes
    that have "helpfully" moved pcap.h to <pcap/pcap.h> without leaving
    behind a <pcap.h> file.