util/AllocatedString: add typedef size_type

src/util/AllocatedString.hxx

@@ -48,6 +48,7 @@ public:
 	typedef typename StringPointer<T>::const_reference_type const_reference_type;
 	typedef typename StringPointer<T>::pointer_type pointer_type;
 	typedef typename StringPointer<T>::const_pointer_type const_pointer_type;
+	typedef size_t size_type;
 
 	static constexpr value_type SENTINEL = '\0';
 
@@ -91,7 +92,7 @@ public:
 	}
 
 	static AllocatedString Duplicate(const_pointer_type begin,
-					 size_t length) {
+					 size_type length) {
 		auto p = new value_type[length + 1];
 		*std::copy_n(begin, length, p) = SENTINEL;
 		return Donate(p);