Add the required c flags for some test executables

Change-Id: I93b0c346fff162899e5f97dd2366e01cf99ec38e
Reviewed-on: https://code.wireshark.org/review/5435
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>