zebra: fix build with rtadv disabled

Otherwise we get warning on rtadv_init() prototype not being
defined when compiling rtadv.c (as dummy stub is provided always).

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
1 file changed