zebra: silence zebra_serv_un unused warning

zebra_serv_un() is unused if --enable-tcp-zebra is given.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/zserv.c b/zebra/zserv.c
index afd722a..432c318 100644
--- a/zebra/zserv.c
+++ b/zebra/zserv.c
@@ -1558,7 +1558,7 @@
 
   zebra_event (ZEBRA_SERV, accept_sock, NULL);
 }
-#endif /* HAVE_TCP_ZEBRA */
+#else /* HAVE_TCP_ZEBRA */
 
 /* For sockaddr_un. */
 #include <sys/un.h>
@@ -1624,6 +1624,7 @@
 
   zebra_event (ZEBRA_SERV, sock, NULL);
 }
+#endif /* HAVE_TCP_ZEBRA */
 
 
 static void