Incorrect platform detection

https://github.com/msoulier/mikelibc/blob/aa926ff3550eee75854b39173958801945cdbe61/mlogger.c#LL7C3-L7C3

`#elif WIN32` is incorrect, and should be replaced with `#elif defined(WIN32)`

In addition, `WIN32` is not guaranteed to work on all Windows platforms. `_WIN32` should be used instead.