flatpak/org.gnome.Photos.json: Fix flatpak build issue with exiv2
flatpak build fail with an error while downloading the exiv2 source files.
Failed to download sources: module exiv2: The requested URL returned error: 404 Not Found
This also break the flatpak pipeline executed during merge request. The issue is caused by the exiv2 repository, where the pointed tar.gz was removed.
Replaced the exiv2 module URL in the flatpak json.