util/{Const,Writable}Buffer: use std::size_t

src/util/StaticFifoBuffer.hxx

@@ -34,9 +34,9 @@
 
 #include <utility>
 #include <algorithm>
+#include <cstddef>
 
 #include <assert.h>
-#include <stddef.h>
 
 /**
  * A first-in-first-out buffer: you can append data at the end, and
@@ -46,7 +46,7 @@
 template<class T, size_t size>
 class StaticFifoBuffer {
 public:
-	using size_type = size_t;
+	using size_type = std::size_t;
 	using Range = WritableBuffer<T>;
 
 protected: