flatpak: Fix the exiv2 build
The exiv2-0.27.1-Source.tar.gz file doesn't exist at it's earlier location. Instead, http://exiv2.org/download.html links to the 0.27.2 release.
The exiv2-0.27.1-Source.tar.gz file doesn't exist at it's earlier location. Instead, http://exiv2.org/download.html links to the 0.27.2 release.