Zoning: Add mode to show restricted signals.

src/zoning_gui.cpp

@@ -40,6 +40,7 @@ static const StringID _zone_type_strings[] = {
 	STR_ZONING_STA_CATCH,
 	STR_ZONING_BUL_UNSER,
 	STR_ZONING_IND_UNSER,
+	STR_ZONING_TRACERESTRICT,
 	INVALID_STRING_ID
 };
 
@@ -50,6 +51,7 @@ static const ZoningEvaluationMode _zone_type_modes[] = {
 	ZEM_STA_CATCH,
 	ZEM_BUL_UNSER,
 	ZEM_IND_UNSER,
+	ZEM_TRACERESTRICT,
 };
 
 static ZoningEvaluationMode DropDownIndexToZoningEvaluationMode(int index)