Make input graph for LSUP_graph_print constant.

include/graph.h

@@ -411,7 +411,7 @@ LSUP_graph_iter_free (LSUP_GraphIterator *it);
  * @param[in] gr Graph handle to display.
  */
 void
-LSUP_graph_print (LSUP_Graph *gr);
+LSUP_graph_print (const LSUP_Graph *gr);
 
 
 /** @brief Get term pairs connected to a term in a graph.

src/graph.c

@@ -651,7 +651,7 @@ LSUP_graph_contains (const LSUP_Graph *gr, const LSUP_Triple *spo)
 
 
 void
-LSUP_graph_print (LSUP_Graph *gr)
+LSUP_graph_print (const LSUP_Graph *gr)
 {
     size_t ct;
     LSUP_GraphIterator *it = LSUP_graph_lookup (gr, NULL, NULL, NULL, &ct);