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.h b/zebra/rtadv.h
index 76f98cf..160814b 100644
--- a/zebra/rtadv.h
+++ b/zebra/rtadv.h
@@ -27,7 +27,7 @@
 #include "zebra/interface.h"
 
 /* NB: RTADV is defined in zebra/interface.h above */
-#ifdef RTADV
+#if defined (HAVE_RTADV)
 
 /* Router advertisement prefix. */
 struct rtadv_prefix
@@ -98,7 +98,7 @@
 
 extern const char *rtadv_pref_strs[];
 
-#endif /* RTADV */
+#endif /* HAVE_RTADV */
 
 extern void rtadv_init (struct zebra_vrf *);
 extern void rtadv_terminate (struct zebra_vrf *);