Add unused-const-variable (enable by default on new clang release)

Change-Id: I1d882e02188ba38ec1b5583c5933d0c92f0539e4
Reviewed-on: https://code.wireshark.org/review/5068
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Balint Reczey <balint@balintreczey.hu>