[panel] Fix 'Invalid write to global property "prefix"' error

The adjustPrefix() function did not specify whose prefix property it was
writing to, so it ended up trying and failing to write to a global one.

This fixes the following error every time plasmashell is launched:
```
file:///home/nate/kde/usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Panel.qml:207: Error: Invalid write to global property "prefix"
```


(cherry picked from commit 1c7e2f39)