Add `po/` to .gitignore
Explicitly prevent KDE l10n scripty from copying translation files to po/. We already copy the translations to locale/ using the StaticMessage.sh script, which also have extra logic to unflatten the directory structure.
Ref: https://mail.kde.org/pipermail/kde-devel/2022-October/001402.html
(Cherry-pick to krita/5.1 and delete po/ from there.)