Fix C4245 warning on MSVC

[tuokri]

Fixes the following warning when compiling on MSVC:

dpp\include\dpp\coro/awaitable.h(686): warning C4245: 'argument': conversion from 'int' to 'const unsigned char', signed/unsigned mismatch

P.S. thanks to Discord user MacOS master.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[Jaskowicz1]

LGTM!