TBTR: Fix incorrect train not buildable warning with articulated units

src/tbtr_template_gui_create.cpp

@@ -306,7 +306,7 @@ public:
 
 					int y = 4 - this->vscroll->GetPosition();
 					bool buildable = true;
-					for (Train *train = this->virtual_train; train != nullptr; train = train->Next()) {
+					for (Train *train = this->virtual_train; train != nullptr; train = train->GetNextUnit()) {
 						if (!IsEngineBuildable(train->engine_type, VEH_TRAIN, train->owner)) buildable = false;
 					}
 					if (!buildable) {

src/tbtr_template_gui_main.cpp

@@ -656,7 +656,7 @@ public:
 			}
 
 			bool buildable = true;
-			for (const TemplateVehicle *u = v; u != nullptr; u = u->Next()) {
+			for (const TemplateVehicle *u = v; u != nullptr; u = u->GetNextUnit()) {
 				if (!IsEngineBuildable(u->engine_type, VEH_TRAIN, u->owner)) {
 					buildable = false;
 					break;