Fix dtrace tools bootstrap on non-FreeBSD after OpenZFS import

[arichardson]

Summary

This modifies one of the contrib headers but I can't see a better solution.

Test Plan

compiles again after this change on Ubuntu and macOS

[mahrens]

Could we change the contrib files upstream (OpenZFS), rather than maintaining the diffs?

[arichardson]

In D26193#582057, @mahrens wrote:

Could we change the contrib files upstream (OpenZFS), rather than maintaining the diffs?

Yeah that would be ideal, just checking if the FreeBSD zfs developers are fine with this ugly change.

[freqlabs]

Looks reasonably noninvasive. I'm surprised it's that easy :)

[arichardson]

The contrib changes were upstream in https://github.com/openzfs/zfs/pull/10863. Is it safe to commit this now or will it cause any issues with the next import (it's exactly the same diff)?

[arichardson]

ping? Are you planning to update the openzfs code soon or is it okay to commit this?