src/libutil/json.cc: add missing <cstdint> include for gcc-13
src/libutil/json.cc: add missing <cstdint> include for gcc-13

Without the change llvm build fails on this week's gcc-13 snapshot as:

    src/libutil/json.cc: In function 'void nix::toJSON(std::ostream&, const char*, const char*)':
    src/libutil/json.cc:33:22: error: 'uint16_t' was not declared in this scope
       33 |             put(hex[(uint16_t(*i) >> 12) & 0xf]);
          |                      ^~~~~~~~
    src/libutil/json.cc:5:1: note: 'uint16_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
        4 | #include <cstring>
      +++ |+#include <cstdint>
        5 |

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