MinGW: add mkdir adapter (#2018)

MinGW: add mkdir adapter (#2018)

On Windows, mkdir only takes one argument.
compat/mswindows.h has an adapter, add it to
compat/mingw.h as well.

Solves error:

```
UFSSwapDir.cc:617:26: error: too many arguments
    to function 'int mkdir(const char*)'
mingw/include/io.h:282:15: note: declared here
    int __cdecl mkdir (const char *);
```