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.
3 jobs for fix_flatpak_build_exiv2 in 14 minutes and 33 seconds
latest
Status Job ID Name Coverage
  Test
passed #304817
flatpak

00:14:06

 
  Review
passed #304818
review

00:00:26

manual #304819
allowed to fail manual
stop_review