MinGW: fix aio compatibility layer (#2020)

MinGW: fix aio compatibility layer (#2020)

The AIO Windows compatibilty layer is also
necessary on mingw

Problems fixed:

```
DiskIO/AIO/async_io.h:58:18:
    error: field 'aq_e_aiocb' has incomplete type 'aiocb'
DiskIO/AIO/async_io.h:58:12:
     note: forward declaration of 'struct aiocb'

DiskIO/AIO/AIODiskFile.cc:
    In member function
    'virtual void AIODiskFile::read(ReadRequest*)':
src/DiskIO/AIO/AIODiskFile.cc:134:9:
    error: 'aio_read' was not declared in this scope;
    did you mean 'file_read' ?
```