Fix compilation error when SA_TRACE is 1

Summary:
The error itself:

```
/home/vlad/Workspace/KDE/src/kde/workspace/libksysguard/ksgrd/SensorAgent.cpp: In member function ‘void KSGRD::SensorAgent::executeCommand()’:
/home/vlad/Workspace/KDE/src/kde/workspace/libksysguard/ksgrd/SensorAgent.cpp:210:60: error: ‘class QString’ has no member named ‘toAscii’
     qCDebug(LIBKSYSGUARD_KSGRD) << ">> " << req->request().toAscii() << "(" << mInputFIFO.count()
                                                            ^~~~~~~
ninja: build stopped: subcommand failed.
```

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19971