cmake: always enable -Qunused-arguments for clang

cmake: always enable -Qunused-arguments for clang

"argument unused during compilation" warnings suggest that some options
do not make sense (such as passing the -gsplit-dwarf option to the
linking command while it is only valid during compilation).

For now we do not care about these warnings, the option is accepted by
clang and whether it used in a particular step or not is uninteresting.

Change-Id: I3b6efa0dccd33c7f351c5938a29adef64ac1706d
Reviewed-on: https://code.wireshark.org/review/10893
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>