cpp/sopt/config.in.h: Add the missing #include <cstdint>

[AdrianBunk]

This fixes a build failure with gcc 13:

In file included from /build/1st/sopt-3.0.1+dfsg/cpp/sopt/../sopt/mpi/session.h:4,
                 from /build/1st/sopt-3.0.1+dfsg/cpp/sopt/mpi/session.cc:1:
/build/1st/sopt-3.0.1+dfsg/obj-x86_64-linux-gnu/include/sopt/config.h:32:19: error: 'uint8_t' was not declared in this scope
   32 | inline std::tuple<uint8_t, uint8_t, uint8_t> version_tuple() {
      |                   ^~~~~~~
/build/1st/sopt-3.0.1+dfsg/obj-x86_64-linux-gnu/include/sopt/config.h:27:1: note: 'uint8_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
   26 | #include <tuple>
  +++ |+#include <cstdint>
   27 | 

[20DM]

Apologies, this has dropped off the radar for so long. It looks like this got fixed via another PR in the meantime now, but thanks for this nevertheless!