[BUILD] Fix error ‘uint8_t’ does not name a type with gcc-15

[AngryLoki]

Fixes #3239

Changes

Added missing cstdint header for uint8_t type in api/include/opentelemetry/logs/severity.h.

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

[linux-foundation-easycla]

• ✅login: AngryLoki / (1977081)

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[marcalff]

LGTM, thanks for the fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.16%. Comparing base (d19eb32) to head (1977081).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3240   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
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  Files         198      198           
  Lines        6224     6224           
=======================================
  Hits         5487     5487           
  Misses        737      737