util/RuntimeError: add IWYU pragma

src/util/RuntimeError.hxx

 /*
- * Copyright 2013-2017 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2013-2020 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -30,7 +30,7 @@
 #ifndef RUNTIME_ERROR_HXX
 #define RUNTIME_ERROR_HXX
 
-#include <stdexcept>
+#include <stdexcept> // IWYU pragma: export
 #include <utility>
 
 #include <stdio.h>