suppress compiler warning.

src/external/jsonhpp/nlohmann/json.hpp

@@ -18646,7 +18646,7 @@ class serializer
 
         // jump to the end to generate the string from backward,
         // so we later avoid reversing the result
-        buffer_ptr += n_chars;
+        buffer_ptr += static_cast<std::ptrdiff_t>(n_chars);
 
         // Fast int2ascii implementation inspired by "Fastware" talk by Andrei Alexandrescu
         // See: https://www.youtube.com/watch?v=o4-CwDo2zpg