Fix default codec being set to "US-ASCII" in KIO apps

From https://doc.qt.io/qt-5/qloggingcategory.html#Q_LOGGING_CATEGORY-1 :
"The implicitly-defined QLoggingCategory object is created on first use,
in a thread-safe manner."

The original way to explicitly define a static QLoggingCategory will lead
to a call to ucnv_getDefaultName() before QApplication constructor (where
setlocale() is called), thus making QTextCodec::codecForLocale() misbehave.
Here we replace the explicit definition with the Q_LOGGING_CATEGORY macro,
and thus avoid this problem.

BUG: 432406