zebra: fix build with rtadv disabled

Otherwise we get warning on rtadv_init() prototype not being
defined when compiling rtadv.c (as dummy stub is provided always).

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/rtadv.h b/zebra/rtadv.h
index 8cb933e..1e1aec9 100644
--- a/zebra/rtadv.h
+++ b/zebra/rtadv.h
@@ -59,7 +59,6 @@
 };
 
 extern void rtadv_config_write (struct vty *, struct interface *);
-extern void rtadv_init (void);
 
 /* RFC4584 Extension to Sockets API for Mobile IPv6 */
 
@@ -101,4 +100,6 @@
 
 #endif /* RTADV */
 
+extern void rtadv_init (void);
+
 #endif /* _ZEBRA_RTADV_H */