Cast a parameter in the kerberos dissector so it compiles without
warnings on NetBSD.

Change-Id: Id1ab5020fa53656065b0b2438071342eae4f7adb
Reviewed-on: https://code.wireshark.org/review/5987
Petri-Dish: Stephen Fisher <sfisher@sdf.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stephen Fisher <sfisher@sdf.org>