Remove const qualifier in queues_get_displayed

The returned node was theoretically read-only however all the other
nodes accessible do not inherit the qualifier. Since none of the glib
functions use const with Glib, even the read-only ones. There's no use
in having it.

This resolves the warning about g_list_nth_data.

src/queues.c

@@ -45,7 +45,7 @@ void queues_init(void)
 }
 
 /* see queues.h */
-const GList *queues_get_displayed(void)
+GList *queues_get_displayed(void)
 {
         return g_queue_peek_head_link(displayed);
 }

src/queues.h

@@ -24,7 +24,7 @@ void queues_init(void);
  *
  * @return read only list of notifications
  */
-const GList *queues_get_displayed(void);
+GList *queues_get_displayed(void);
 
 /**
  * Get the highest notification in line