net/AllocatedSocketAddress: add missing forward declaration

src/net/AllocatedSocketAddress.hxx

 /*
- * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2020 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
@@ -39,6 +39,7 @@
 #include <stdlib.h>
 
 struct sockaddr;
+struct StringView;
 
 class AllocatedSocketAddress {
 public: