[IPV6] sit: Add missing MODULE_LICENSE

This is missing the MODULE_LICENSE statements and taints the kernel
upon loading. License is obvious from the beginning of the file.

Signed-off-by: Jan Dittmer <jdi@l4x.org>
Signed-off-by: Joerg Roedel <joro-lkml@zlug.org>
Signed-off-by: David S. Miller <davem@davemloft.net>