selinux: nlmsgtab: add SOCK_DESTROY to the netlink mapping tables

Without this, using SOCK_DESTROY in enforcing mode results in:

  SELinux: unrecognized netlink message type=21 for sclass=32

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>