Fix build warnings under rustc 1.75.0

[jkczyz]

No description provided.

[ldk-reviews-bot]

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[jkczyz]

Locally, I see a doc test failing to compile under c_bindings, but I'm not sure how to resolve it. Using conditional code compilation doesn't seem to work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (08c2236) to head (2338805).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4212      +/-   ##
==========================================
+ Coverage   89.30%   89.31%   +0.01%     
==========================================
  Files         180      180              
  Lines      137913   137913              
  Branches   137913   137913              
==========================================
+ Hits       123159   123174      +15     
+ Misses      12142    12133       -9     
+ Partials     2612     2606       -6     

Flag	Coverage Δ
fuzzing	33.60% <0.00%> (+0.09%)	⬆️
tests	88.71% <100.00%> (+<0.01%)	⬆️

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[tnull]

Simple enough. This was also fixed in #4175, but no reason not to already land this.

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