selinux/nlmsg: add a build time check for rtnl/xfrm cmds

When a new rtnl or xfrm command is added, this part of the code is frequently
missing. Let's help the developer with a build time test.

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