std.windows.syserror: Fix doc build error

#2616 already incorporates this fix, but since it's not ready for merging, this is a quick fix until then.

The error this fixes:
```
std\file.d(1503): Error: safe function 'std.file.mkdir' cannot call system function 'std.windows.syserror.wenforce!(int, const(char)[]).wenforce'
```