Qt: make parameter unused when SHOW_BUFFER_COLUMN is undefined.

Change-Id: I87843f61d4c98fea90e49088f3d56df5aa801e58
Reviewed-on: https://code.wireshark.org/review/17946
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Dario Lombardo <lomato@gmail.com>

ui/qt/capture_interfaces_dialog.cpp

@@ -1297,6 +1297,8 @@ void InterfaceTreeDelegate::bufferSizeChanged(int value)
         return;
     }
     device->buffer = value;
+#else
+    Q_UNUSED(value);
 #endif
 }