bgp: compiler warning fix

* bgp_filter.h: Gcc complains the function prototype is not correct because
  the function argument is using old K&R style.

bgpd/bgp_filter.h

@@ -33,7 +33,7 @@ extern void bgp_filter_reset (void);
 extern enum as_filter_type as_list_apply (struct as_list *, void *);
 
 extern struct as_list *as_list_lookup (const char *);
-extern void as_list_add_hook (void (*func) ());
-extern void as_list_delete_hook (void (*func) ());
+extern void as_list_add_hook (void (*func) (void));
+extern void as_list_delete_hook (void (*func) (void));
 
 #endif /* _QUAGGA_BGP_FILTER_H */