thumbnailer: Always initialise locale on startup
So that thumbnailers can print debug in UTF-8.
Updated from gnome-thumbnailer-skeleton module: https://github.com/hadess/gnome-thumbnailer-skeleton/commit/90ce167c815dbe5064b64f1165814797bae80c50
Should probably be backported if needed, as some files might not be accessible otherwise (gnome-desktop!104 (merged))