replace include guards with pragma once

src/graphics/ui/gui_xml.hpp

@@ -1,5 +1,4 @@
-#ifndef FRONTEND_GUI_GUI_XML_HPP_
-#define FRONTEND_GUI_GUI_XML_HPP_
+#pragma once
 
 #include "GUI.hpp"
 #include "coders/xml.hpp"
@@ -56,5 +55,3 @@ namespace gui {
         const std::string& getFilename() const;
     };
 }
-
-#endif // FRONTEND_GUI_GUI_XML_HPP_