applications/plasma-mobile: Add fetch script

Used similarly to the KDE Gear scripts:

```
$ ./maintainers/scripts/fetch-kde-qt.sh pkgs/applications/plasma-mobile/
```

pkgs/applications/plasma-mobile/fetch.sh

···
+WGET_ARGS=( http://download.kde.org/stable/plasma-mobile/21.05 -A '*.tar.xz' )