wireshark: set mainProgram

It'd try to run wireshark-qt or wireshark-cli otherwise.

pkgs/applications/networking/sniffers/wireshark/default.nix

···
 
     platforms = platforms.linux ++ platforms.darwin;
     maintainers = with maintainers; [ bjornfor fpletz ];
+    mainProgram = if withQt then "wireshark" else "tshark";
   };
 }