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.)
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