zebra: use prototypes/includes in rt_netlink

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/if_netlink.c b/zebra/if_netlink.c
index 86bd8ff..8701416 100644
--- a/zebra/if_netlink.c
+++ b/zebra/if_netlink.c
@@ -23,12 +23,11 @@
 #include <zebra.h>
 
 #include "zebra/zserv.h"
-
-extern int interface_lookup_netlink (void);
+#include "rt_netlink.h"
 
 /* Interface information read by netlink. */
 void
 interface_list (void)
 {
-  interface_lookup_netlink ();  
+  interface_lookup_netlink ();
 }