ExportProjectDialog: fixed broken cancel button

The cancel button functionality was lost during introduction of the
multi track export functionality.

Closes #3598342.

src/gui/export_project_dialog.cpp

@@ -1,7 +1,7 @@
 /*
  * export_project_dialog.cpp - implementation of dialog for exporting project
  *
- * Copyright (c) 2004-2012 Tobias Doerffel <tobydox/at/users.sourceforge.net>
+ * Copyright (c) 2004-2013 Tobias Doerffel <tobydox/at/users.sourceforge.net>
  *
  * This file is part of Linux MultiMedia Studio - http://lmms.sourceforge.net
  *
@@ -103,6 +103,8 @@ void exportProjectDialog::reject()
 	{
 		(*it)->abortProcessing();
 	}
+
+	QDialog::reject();
 }