Use const for nng_stat when possible.

[gdamore]

This should help the compiler enforce checks, and may result in better optimizations.

fixes #

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[codecov]

Codecov Report

Attention: Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.

Project coverage is 80.22%. Comparing base (5830408) to head (6937731).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/stats.c	57.89%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1895      +/-   ##
==========================================
+ Coverage   80.07%   80.22%   +0.14%     
==========================================
  Files          95       95              
  Lines       25010    25011       +1     
  Branches     3395     3395              
==========================================
+ Hits        20026    20064      +38     
+ Misses       4912     4874      -38     
- Partials       72       73       +1     

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