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/rtadv.h b/zebra/rtadv.h
index 564a4c6..8cb933e 100644
--- a/zebra/rtadv.h
+++ b/zebra/rtadv.h
@@ -26,6 +26,9 @@
 #include "vty.h"
 #include "zebra/interface.h"
 
+/* NB: RTADV is defined in zebra/interface.h above */
+#ifdef RTADV
+
 /* Router advertisement prefix. */
 struct rtadv_prefix
 {
@@ -96,4 +99,6 @@
 
 extern const char *rtadv_pref_strs[];
 
+#endif /* RTADV */
+
 #endif /* _ZEBRA_RTADV_H */