sit: fix panic with route cache in ip tunnels

Bug introduced by commit 7d442fab ("ipv4: Cache dst in tunnels").

Because sit code does not call ip_tunnel_init(), the dst_cache was not
initialized.

CC: Tom Herbert <therbert@google.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>