fix for image load issues on Windows

I have received this message in iNaturalist:

From langzi:

Hello, I'm a new user of Digikam and I'm very excited about your plugin for exporting to inat. I just want to tell you that under windows 10, when I click in the 'enter identification' field I get an error message saying 'unknown error' and afterwards I can still choose a species from the id field but the export does not work. That's it, thanks for your work!

The issue is that photos cannot be loaded at all by the plugin on Windows. They are attempted to load with '/' instead of '\' in the filenames. If that different separator is the root cause, this change will fix the issue.