org.kde.dolphin.appdata: Add developer_name

Fixes the following lint errors for Flathub:
```
{
    "errors": [
        "appstream-missing-developer-name"
    ]
}
```

See: flathub/org.kde.dolphin#207
See: flathub-infra/flatpak-builder-lint#274
See: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer
See: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name

src/org.kde.dolphin.appdata.xml

@@ -3,6 +3,7 @@
   <id>org.kde.dolphin.desktop</id>
   <metadata_license>CC0-1.0</metadata_license>
   <project_license>GPL-2.0-or-later</project_license>
+  <developer_name translate="no">KDE</developer_name>
   <name>Dolphin</name>
   <name xml:lang="ar">دولفين</name>
   <name xml:lang="ast">Dolphin</name>