Add dissector table for sFlow header dissection.

Change-Id: I189a5b2d5f769395fa5e387f8be49b81903fb252
Reviewed-on: https://code.wireshark.org/review/5262
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>