Define a function prototype for YieldProcessor

[jkoritzinsky]

Fixes official build failures.

[am11]

I think minipal deserves its own label. 🏷️

[am11]

It must be due to inline otherwise none of the helpers required separate prototype https://github.com/dotnet/runtime/blob/6c7ece9479c7cbb0ca6e372adf0a0154a5f0c1a0/src/native/minipal/utf8.c. We can simply drop the inline and let compiler decide (inline is just a hint and compiler can choose to ignore it if present and vice versa).