util, io, net, ...: use "#pragma once"

src/util/Exception.hxx

@@ -1,8 +1,7 @@
 // SPDX-License-Identifier: BSD-2-Clause
 // author: Max Kellermann <max.kellermann@gmail.com>
 
-#ifndef EXCEPTION_HXX
-#define EXCEPTION_HXX
+#pragma once
 
 #include <exception>
 #include <string>
@@ -113,5 +112,3 @@ std::string
 GetFullMessage(std::exception_ptr ep,
 	       const char *fallback="Unknown exception",
 	       const char *separator="; ") noexcept;
-
-#endif