missing: define NET_NAME_UNKNOWN

It's only exposed to userspace since

  commit 685343fc3ba61a1f6eef361b786601123db16c28
  Author:     Tom Gundersen <teg@jklm.no>
  AuthorDate: Mon Jul 14 16:37:22 2014 +0200
  Commit:     David S. Miller <davem@davemloft.net>
  CommitDate: Tue Jul 15 16:12:01 2014 -0700

to the kernel.