Expose the select_rolling_resample_op function (#2017)

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### Pull Request Checklist:
- [ ] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #xyz
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [ ] CHANGELOG.rst has been updated (with summary of main changes)
- [ ] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

* The function was added in #1643, but I just realized that I forgot to
expose it in the `__all__`. Thus, it does not appear anywhere in the
xclim documentation.

### Does this PR introduce a breaking change?


### Other information:

src/xclim/indices/generic.py

@@ -55,6 +55,7 @@
     "last_occurrence",
     "season",
     "select_resample_op",
+    "select_rolling_resample_op",
     "spell_length",
     "spell_length_statistics",
     "spell_mask",