yang: add missing zif-gre zebra interface type (backport #15803)

[mergify]

This was caught by the grpc_basic test which was receiving an invalid error result, which was returned b/c inside zebra the libyang code was flagging the value as invalid for a derived zebra interface type.

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This is an automatic backport of pull request #15803 done by Mergify.

[mergify]

Cherry-pick of 74fb405 has failed:

On branch mergify/bp/stable/10.0/pr-15803
Your branch is up to date with 'origin/stable/10.0'.

You are currently cherry-picking commit 74fb4056c.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   yang/frr-zebra.yang

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   zebra/zebra_nb_state.c

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally