[eclass] Correct the destination path for various config files

Currently, at least the following packages install files to `/usr/etc` instead of `/etc`:
```
kde-base/kinfocenter
kde-base/kio-extras
kde-base/ksysguard
kde-base/kwin
kde-base/kwrited
kde-base/plasma-desktop
kde-base/plasma-workspace
kde-frameworks/khtml
kde-frameworks/kio
kde-frameworks/ktexteditor
kde-frameworks/kxmlgui
```

This PR fixes that. Needs a manual rebuild of affected packages:
```
emerge -1 /usr/etc
```