stable_headers: Move to pragma once

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

src/stable_headers.h

-#ifndef STABLE_HEADERS_H
-#define STABLE_HEADERS_H
+#pragma once
 
 #include <QtCore>
 #include <QtGui>
 #include <QtWidgets>
 
-#endif // STABLE_HEADERS_H