ospf6d: fix compile warning from falling off end of main

ospf6 main always calls ospf6_exit() which always calls exit.
Use attributes to tell GCC that this is ok.
diff --git a/ospf6d/ospf6_main.c b/ospf6d/ospf6_main.c
index 0e61315..6f7717c 100644
--- a/ospf6d/ospf6_main.c
+++ b/ospf6d/ospf6_main.c
@@ -129,7 +129,7 @@
   exit (status);
 }
 
-static void
+static void __attribute__ ((noreturn))
 ospf6_exit (int status)
 {
   extern struct ospf6 *ospf6;