QLoggingRegistry: mark that qtLoggingDebug() is unlikely to be true

Just to nudge the compilers in the right way of optimizing.

Change-Id: I3cf0c3541326c0aec113fffd96a75d3b18f8162d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/io/qloggingregistry.cpp

@@ -256,7 +256,7 @@ static bool qtLoggingDebug()
             debugMsg("QT_LOGGING_DEBUG environment variable is set.");
         return debug;
     }();
-    return debugEnv;
+    return Q_UNLIKELY(debugEnv);
 }
 
 static QList<QLoggingRule> loadRulesFromFile(const QString &filePath)