QGCApplication: Use forward declaration in QQmlApplicationEngine

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

src/QGCApplication.h

@@ -11,7 +11,6 @@
 
 #include <QApplication>
 #include <QTimer>
-#include <QQmlApplicationEngine>
 #include <QElapsedTimer>
 
 #include "LinkConfiguration.h"
@@ -31,6 +30,7 @@
 #endif
 
 // Work around circular header includes
+class QQmlApplicationEngine;
 class QGCSingleton;
 class QGCToolbox;
 class QGCFileDownload;