add covering default case in Tile::AssembleTile

temporary - replace with case flow_type::NONE after DFHack/df-structures#872 is merged

Author: ab9rf

GraphicsWrapper.cpp

@@ -678,7 +678,9 @@ void Tile::AssembleTile( void )
         case df::flow_type::ItemCloud:
             // TODO
             break;
-
+        default:
+            assert("Unhandled flow type" && false); // FIXME temporary - replace with NONE case after structures are updated
+            break;
         }
     }