Add setting to increase cost/distance of aircraft linkgraph links

src/settings_type.h

@@ -691,6 +691,7 @@ struct LinkGraphSettings {
 	uint8 demand_size;                          ///< influence of supply ("station size") on the demand function
 	uint8 demand_distance;                      ///< influence of distance between stations on the demand function
 	uint8 short_path_saturation;                ///< percentage up to which short paths are saturated before saturating most capacious paths
+	uint16 aircraft_link_scale;                 ///< scale effective distance of aircraft links
 
 	inline DistributionType GetDistributionType(CargoID cargo) const {
 		if (this->distribution_per_cargo[cargo] != DT_PER_CARGO_DEFAULT) return this->distribution_per_cargo[cargo];