Address the cpplint findings of json_view's materialize()

materialize.hpp includes <string> (build/include_what_you_use).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/materialize.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <cstdint> // int64_t, uint8_t
+#include <string> // string
 #include <vector> // vector
 
 #include <nlohmann/json.hpp>