policies: add firstnodex to the known policies

[trws]

problem: the firstnodex policy was added and tested, but couldn't be selected by users because it wouldn't be seen as a known policy.

solution: accept the firstnodex value as a known policy in known_match_policy

[codecov[bot]]

Codecov Report

Merging #1161 (a587a97) into master (d0b9ac3) will not change coverage. The diff coverage is 100.0%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1161 +/- ## ====================================== Coverage 73.9% 73.9% ====================================== Files 103 103 Lines 14395 14395 ====================================== Hits 10643 10643 Misses 3752 3752 ``` | [Files](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1161?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework) | Coverage Δ | | |---|---|---| | [resource/policies/dfu\_match\_policy\_factory.cpp](https://app.codecov.io/gh/flux-framework/flux-sched/pull/1161?src=pr&el=tree&filepath=resource%2Fpolicies%2Fdfu_match_policy_factory.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flux-framework#diff-cmVzb3VyY2UvcG9saWNpZXMvZGZ1X21hdGNoX3BvbGljeV9mYWN0b3J5LmNwcA==) | `92.8% <100.0%> (ø)` | |