Make LSUP_graph_uri return const.

include/graph.h

@@ -139,7 +139,7 @@ LSUP_graph_equals (const LSUP_Graph *gr1, const LSUP_Graph *gr2);
  *
  * To change the graph URI, use #LSUP_graph_set_uri.
  */
-LSUP_Term *
+const LSUP_Term *
 LSUP_graph_uri (const LSUP_Graph *gr);
 
 

src/graph.c

@@ -201,7 +201,7 @@ LSUP_graph_free (LSUP_Graph *gr)
 }
 
 
-LSUP_Term *
+const LSUP_Term *
 LSUP_graph_uri (const LSUP_Graph *gr) { return gr->uri; }