Add setting to only spawn primary industries

src/settings_type.h

@@ -736,6 +736,7 @@ struct EconomySettings {
 	bool   give_money;                       ///< allow giving other companies money
 	bool   mod_road_rebuild;                 ///< roadworks remove unnecessary RoadBits
 	bool   multiple_industry_per_town;       ///< allow many industries of the same type per town
+	bool   spawn_primary_industry_only;      ///< only spawn primary industried
 	int8_t   town_growth_rate;               ///< town growth rate
 	uint8_t  town_growth_cargo_transported;  ///< percentage of town growth rate which depends on proportion of transported cargo in the last month
 	bool   town_zone_calc_mode;              ///< calc mode for town zones