Create a main include file QExtSerialPort

This main include file will be installed in
<QExtSerialPort/QExtSerialPort> so that Qt applications can use this
library by including header files in a Qt-like style.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: qextserialport-ef4af2a2ee3f/src/QExtSerialPort
===================================================================
--- /dev/null
+++ qextserialport-ef4af2a2ee3f/src/QExtSerialPort
@@ -0,0 +1,2 @@
+#include "qextserialport.h"
+#include "qextserialenumerator.h"