FreeBSD also needs libgen.h

(closes issue #14051)
Reported by: ys
Patches:
      res_http_post.c.diff uploaded by ys (license 281)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@162660 65c4cc65-6c06-0410-ace0-fbb531ad65f3