Replace include guards with #pragma once

Change-Id: Ic36cf0e9cac61d4d9a7e4434192944f5301b8aaa
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/imageviewer/exportdialog.h

@@ -23,8 +23,7 @@
 **
 ****************************************************************************/
 
-#ifndef EXPORTDIALOG_H
-#define EXPORTDIALOG_H
+#pragma once
 
 #include <QDialog>
 
@@ -66,5 +65,3 @@ private:
 
 } // namespace Internal
 } // namespace ImageViewer
-
-#endif // EXPORTDIALOG_H