zebra: fix up compilation without rtadv/IPv6

let's ground the rtadv.h file if route advertisements are disabled. And
fix up the CLI for it, as well as move the "show ip mroute" to its
proper place.

  * zebra/rtadv.h: #ifdef RTADV
  * zebra/main.c: #ifdef RTADV
  * zebra/zebra_vty.c: move "show ip mroute" out of #ifdef IPV6

From: Joachim Nilsson <troglobit@gmail.com>
[moved #ifdef RTADV to rtadv.h]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/main.c b/zebra/main.c
index 5f26ce2..5b5265f 100644
--- a/zebra/main.c
+++ b/zebra/main.c
@@ -327,7 +327,9 @@
   zebra_vty_init ();
   access_list_init ();
   prefix_list_init ();
+#ifdef RTADV
   rtadv_init ();
+#endif
 #ifdef HAVE_IRDP
   irdp_init();
 #endif