Revamp the Enumerator Example

* Update includes to common style.
* Remove QT_USE_NAMESPACE macro.

Task-number: QTBUG-60652
Change-Id: Ic8860e7e6d0fa320e251e02f3d294845c662a8d0
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

examples/serialport/enumerator/main.cpp

@@ -50,13 +50,11 @@
 ****************************************************************************/
 
 #include <QApplication>
-#include <QWidget>
-#include <QVBoxLayout>
 #include <QLabel>
 #include <QScrollArea>
-#include <QtSerialPort/QSerialPortInfo>
-
-QT_USE_NAMESPACE
+#include <QSerialPortInfo>
+#include <QVBoxLayout>
+#include <QWidget>
 
 int main(int argc, char *argv[])
 {