[BUILD] Fix variable shadow

[owent]

Fixes #2495

Changes

• Fix variable shadow in some old compilers.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2b5c2b4) 87.08% compared to head (ac5d680) 87.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2498      +/-   ##
==========================================
+ Coverage   87.08%   87.09%   +0.02%     
==========================================
  Files         200      200              
  Lines        6103     6103              
==========================================
+ Hits         5314     5315       +1     
+ Misses        789      788       -1     

Files	Coverage Δ
api/include/opentelemetry/nostd/function_ref.h	77.28% <100.00%> (ø)

... and 1 file with indirect coverage changes

[marcalff]

LGTM, thanks for the fix