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

[trofi]

Without the change nix_2_3 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 |

(cherry picked from commit b36d517)

Motivation

Context

Priorities

Add 👍 to pull requests you find important.