Add #include <cctype> to baggage.h

[lidavidm]

Fixes #808

Changes

Add #include <cctype> to make isdigit, isalnum, etc. available in baggage.h.

I also tested against the Abseil-variant branch and confirmed that 1) without this, we still get errors under MSVC2015 and 2) with this, there are no errors.

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

[codecov]

Codecov Report

Merging #822 (9e3b0f9) into main (0f6199f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #822   +/-   ##
=======================================
  Coverage   96.19%   96.19%           
=======================================
  Files         153      153           
  Lines        6444     6444           
=======================================
  Hits         6198     6198           
  Misses        246      246           

Impacted Files	Coverage Δ
api/include/opentelemetry/baggage/baggage.h	97.30% <ø> (ø)

[maxgolov]

Thank you for the fix. We'd consider adding some tests / samples. Perhaps it's one of these extended features that we are not compiling in any of the examples. I'll set up the build loop for vs2015 once the Abseil branch changes get merged this week, to continuously validate everything with vs2015.