fix compilation with gcc7

See https://forum.polymake.org/viewtopic.php?f=8&t=559
---
 lib/core/include/Graph.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/lib/core/include/Graph.h b/lib/core/include/Graph.h
index 736257fe1e..be40a42c31 100644
--- a/lib/core/include/Graph.h
+++ b/lib/core/include/Graph.h
@@ -709,8 +709,8 @@ public:
    void swap(Table& t)
    {
       std::swap(R,t.R);
-      std::swap(node_maps, t.node_maps);
-      std::swap(edge_maps, t.edge_maps);
+      node_maps.swap(t.node_maps);
+      edge_maps.swap(t.edge_maps);
       std::swap(n_nodes, t.n_nodes);
       std::swap(free_node_id, t.free_node_id);
       std::swap(free_edge_ids, t.free_edge_ids);
-- 
2.13.0

