src: add #include <string> in json_utils.h

When precompile headers are disabled this file would cause compilation
errors due to #include <string> not included.

PR-URL: https://github.com/nodejs/node/pull/33332


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>