Add missing hash value function for Seam_mesh's edges

BGL/include/CGAL/boost/graph/Seam_mesh.h

@@ -439,6 +439,11 @@ public:
     {
       return ! (e1 == e2);
     }
+
+    friend std::size_t hash_value(const edge_descriptor ed)
+    {
+      return hash_value((std::min)(ed.hd, ed.mesh_->opposite(ed.hd)));
+    }
   };
 
 #ifndef DOXYGEN_RUNNING