Add road stop property to set cost multipliers

src/newgrf_extension.h

@@ -52,6 +52,7 @@ enum Action0RemapPropertyIds {
 	A0RPI_ROADSTOP_GENERAL_FLAGS,
 	A0RPI_ROADSTOP_MIN_BRIDGE_HEIGHT,
 	A0RPI_ROADSTOP_DISALLOWED_BRIDGE_PILLARS,
+	A0RPI_ROADSTOP_COST_MULTIPLIERS,
 };