ngrep: additional flag prevents segfault

Known issue on BSD platforms since v1.4.4 and
configure produces a warning if not present:

    https://github.com/jpr5/ngrep/commit/e29fc29

NB ngrep seems to have moved to git at
https://github.com/jpr5/ngrep but I didn't get
master to compile successfully so I left it out.