selinux/nlmsg: add XFRM_MSG_MIGRATE

commit 8d465bb777179c4bea731b828ec484088cc9fbc1 upstream (net-next).

This command is missing.

Change-Id: Id2c9344ca1ab2c96e0b758ad1efb38e16cf23b86
Fixes: 5c79de6e ("[XFRM]: User interface for handling XFRM_MSG_MIGRATE")
Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>