Fix Clang-23 RC build

[evantypanski]

draft since I'm going to look at other stuff too (and so I make sure to come back to this), this isn't urgent.

Just tested Clang 23 RC3 and hit an error building Zeek:

In file included from /Users/etyp/src/zeek/auxil/spicy/hilti/runtime/src/backtrace.cc:6:
/Users/etyp/src/zeek/auxil/spicy/hilti/runtime/include/hilti/rt/backtrace.h:57:9: error: use of undeclared identifier 'free'
   57 |         free(dname); // NOLINT
      |         ^~~~
1 error generated.

Also changed this to fix some warnings, as well.

Note this fixes warnings in fiber/ - I did that as patches (see zeek/fiber#1). Unless they respond upstream to the other patch, I don't see much reason to not mildly maintain it ourselves.

[codspeed-hq]

Merging this PR will not alter performance

✅ 43 untouched benchmarks

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_{Comparing topic/etyp/transitive-include-fix (e68109d) with main (9274feb)¹}

Footnotes

1. No successful run was found on main (877b266) during the generation of this report, so 9274feb was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩