remove prototype of non existing function

Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc: Fix missing in6addr_loopback

Reported-by: James Peach <jorgar@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>