PE: Fix compile with older Qt

Otherwise meta object system complains.

Change-Id: Ibb929e7341aac92b8272d9647d25e88bb9668319
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/appoutputpane.cpp

@@ -3,6 +3,7 @@
 
 #include "appoutputpane.h"
 
+#include "project.h"
 #include "projectexplorer.h"
 #include "projectexplorerconstants.h"
 #include "projectexplorericons.h"