[cleanup] functions taking no args should be declared with void args

Use Ansi-C prototypes rather than old K&R method of declaring
function without arguments
diff --git a/zebra/rtadv.c b/zebra/rtadv.c
index e1fd0dc..dd2be81 100644
--- a/zebra/rtadv.c
+++ b/zebra/rtadv.c
@@ -524,7 +524,7 @@
 }
 
 static struct rtadv_prefix *
-rtadv_prefix_new ()
+rtadv_prefix_new (void)
 {
   return XCALLOC (MTYPE_RTADV_PREFIX, sizeof (struct rtadv_prefix));
 }