org.kde.okular.appdata: Add developer_name

Fixes the following lint errors for Flathub:

{
    "errors": [
        "appstream-missing-developer-name"
    ]
}

See:

• https://github.com/flathub/org.kde.kdenlive/pull/329
• https://github.com/flathub-infra/flatpak-builder-lint/issues/274
• https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
• https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name
• https://mail.kde.org/pipermail/kde-devel/2024-February/002400.html