minor fix for code examples (#1317)

diff --git a/example/readFromString/readFromString.cpp b/example/readFromString/readFromString.cpp
index c27bbd5..0b29a4e 100644
--- a/example/readFromString/readFromString.cpp
+++ b/example/readFromString/readFromString.cpp
@@ -1,5 +1,6 @@
 #include "json/json.h"
 #include <iostream>
+#include <memory>
 /**
  * \brief Parse a raw string into Value object using the CharReaderBuilder
  * class, or the legacy Reader class.