zebra: fix up compilation without rtadv/IPv6

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>