zebra: Cleanup RTADV define

The RTADV define was not being set correctly or consistently.
Make the code consistent with our HAVE_IPV6 define.

If the user wants to explicitly turn it off then they should
run --disable-rtadv from the configure cli

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/zebra/rtadv.c b/zebra/rtadv.c
index f5d5b4a..6b49cf6 100644
--- a/zebra/rtadv.c
+++ b/zebra/rtadv.c
@@ -41,7 +41,7 @@
 
 extern struct zebra_privs_t zserv_privs;
 
-#if defined (HAVE_IPV6) && defined (RTADV)
+#if defined (HAVE_IPV6) && defined (HAVE_RTADV)
 
 #ifdef OPEN_BSD
 #include <netinet/icmp6.h>
@@ -1813,4 +1813,4 @@
 {
   /* Empty.*/;
 }
-#endif /* RTADV && HAVE_IPV6 */
+#endif /* HAVE_RTADV && HAVE_IPV6 */