fix: hash_agg bench

[wangrunji0408]

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What's changed and what's your intention?

fix #13181

This line is needed to ensure functions are linked to the binary.

risingwave_expr_impl::enable!();

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
• My PR contains breaking changes. (If it deprecates some features, please create a tracking issue to remove them in the future).
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[kwannoel]

LGTM thanks!

[codecov]

Codecov Report

Merging #13193 (172a080) into main (7122d6c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main   #13193   +/-   ##
=======================================
  Coverage   68.10%   68.10%           
=======================================
  Files        1507     1507           
  Lines      255634   255634           
=======================================
+ Hits       174103   174104    +1     
+ Misses      81531    81530    -1     

Flag	Coverage Δ
rust	68.10% <ø> (+<0.01%)	⬆️

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see 5 files with indirect coverage changes

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