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/main.c b/zebra/main.c
index c5d1d76..f3c08f1 100644
--- a/zebra/main.c
+++ b/zebra/main.c
@@ -229,7 +229,7 @@
 
   assert (zvrf);
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
   rtadv_init (zvrf);
 #endif
   kernel_init (zvrf);
@@ -260,7 +260,7 @@
         if_down (ifp);
     }
 
-#ifdef RTADV
+#if defined (HAVE_RTADV)
   rtadv_terminate (zvrf);
 #endif
   kernel_terminate (zvrf);
@@ -405,7 +405,7 @@
   zebra_vty_init ();
   access_list_init ();
   prefix_list_init ();
-#ifdef RTADV
+#if defined (HAVE_RTADV)
   rtadv_cmd_init ();
 #endif
 #ifdef HAVE_IRDP