Add back -Qunused-arguments for Clang++.

When used with ccache it can result in warnings as described at
http://petereisentraut.blogspot.com/2011/05/ccache-and-clang.html.

Change-Id: Ibca4fd262ec8d1f16a450b1698ead10f57d52b06
Reviewed-on: https://code.wireshark.org/review/3015
Reviewed-by: Gerald Combs <gerald@wireshark.org>