Add a cast to satisfy the buildbots.

Change-Id: I1c128db8a8b0cbbef01a26f7b3a50ed2b59af229
Reviewed-on: https://code.wireshark.org/review/2279
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
Tested-by: Jeff Morriss <jeff.morriss.ws@gmail.com>
(cherry picked from commit 319ad977175e34dde70799aae7c68ff24f24b977)
Reviewed-on: https://code.wireshark.org/review/2922
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Anders Broman <a.broman58@gmail.com>