[IPV6]: Give sit driver an appropriate module alias.

It would be nice to keep things working even with this built as a
module, it took me some time to realize my IPv6 tunnel was broken
because of the missing sit module. This module alias fixes things
until distributions have added an appropriate alias to modprobe.conf.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>