[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" ```
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