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

[zcbenz]

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

d:\a\1\s\node\src\json_utils.h(141): error C2679: binary '<<': no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines