flatpak: Fix the exiv2 build

The exiv2-0.27.0-Source.tar.gz file doesn't exist at it's earlier
location. Instead, http://exiv2.org/download.html links to the 0.27.1
release.

!103
1 job for master in 15 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #313252
flatpak

00:15:43