Include cstdint to fix GNU/Linux build

[ausbin]

This is necessary for building on my machine (Debian 13, gcc 14.2.0).

Without this, I get:

[snip]/qiree/src/qiree/ResultDistribution.hh:67:48: error: ‘std::uint64_t’ has not been declared
   67 | void encode_bit_string(std::string const& key, std::uint64_t& result);
      |                                                ^~~

(Almost too small to be a PR, but starting off small before I send crazier PRs)

[sethrj]

Thanks!