util/{Const,Writable}Buffer: add typedef value_type

src/util/ConstBuffer.hxx

 /*
- * Copyright (C) 2013-2014 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright (C) 2013-2017 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
@@ -44,6 +44,7 @@ struct ConstBuffer;
 template<>
 struct ConstBuffer<void> {
 	typedef size_t size_type;
+	typedef void value_type;
 	typedef const void *pointer_type;
 	typedef pointer_type const_pointer_type;
 	typedef pointer_type iterator;
@@ -86,6 +87,7 @@ struct ConstBuffer<void> {
 template<typename T>
 struct ConstBuffer {
 	typedef size_t size_type;
+	typedef T value_type;
 	typedef const T &reference_type;
 	typedef reference_type const_reference_type;
 	typedef const T *pointer_type;

src/util/WritableBuffer.hxx

 /*
- * Copyright (C) 2013-2014 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright (C) 2013-2017 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
@@ -44,6 +44,7 @@ struct WritableBuffer;
 template<>
 struct WritableBuffer<void> {
 	typedef size_t size_type;
+	typedef void value_type;
 	typedef void *pointer_type;
 	typedef const void *const_pointer_type;
 	typedef pointer_type iterator;
@@ -80,6 +81,7 @@ struct WritableBuffer<void> {
 template<typename T>
 struct WritableBuffer {
 	typedef size_t size_type;
+	typedef T value_type;
 	typedef T &reference_type;
 	typedef const T &const_reference_type;
 	typedef T *pointer_type;