docs: improvements to source code documentation (#1236)

src/system_tray.h

@@ -2,8 +2,7 @@
  * @file system_tray.h
  */
 
-#ifndef SUNSHINE_SYSTEM_TRAY_H
-#define SUNSHINE_SYSTEM_TRAY_H
+#pragma once
 
 // system_tray namespace
 namespace system_tray {
@@ -31,4 +30,3 @@ namespace system_tray {
   end_tray();
 
 }  // namespace system_tray
-#endif  // SUNSHINE_SYSTEM_TRAY_H