Fix https://github.com/OpenSmalltalk/opensmalltalk-vm/issues/531
Fix https://github.com/OpenSmalltalk/opensmalltalk-vm/issues/531

The return type of `currentUTCMicroseconds` was lost.
For some reason, the cygwin/mingw takes this compilation path

`#else // _WIN32_WINNT >= _WIN32_WINNT_WIN8`

OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm