Add missing cstdint include for gcc15

[andrjohns]

s2 is failing to build on a Ubuntu 25.10 system with gcc 15.2, as the cstdint header is no longer transitively included by other headers:

[ 69%] Building CXX object absl/flags/CMakeFiles/flags_reflection.dir/reflection.cc.o
In file included from /usr/include/c++/15/cassert:46,
                 from /tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/container/internal/container_memory.h:18,
                 from /tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/container/flat_hash_map.h:40,
                 from /tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/flags/reflection.h:29,
                 from /tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/flags/reflection.cc:16:
/tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/container/internal/container_memory.h: In function ‘void* absl::s2_lts_20230802::container_internal::Allocate(Alloc*, size_t)’:
/tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/container/internal/container_memory.h:66:27: error: ‘uintptr_t’ does not name a type [-Wtemplate-body]
   66 |   assert(reinterpret_cast<uintptr_t>(p) % Alignment == 0 &&
      |                           ^~~~~~~~~
/tmp/RtmpYafy8t/R.INSTALL3700a7df5279d/s2/tools/vendor/abseil-cpp/absl/container/internal/container_memory.h:31:1: note: ‘uintptr_t’ is defined in header ‘<cstdint>’; this is probably fixable by adding ‘#include <cstdint>’
   30 | #include "absl/utility/utility.h"
  +++ |+#include <cstdint>
   31 | 
make[2]: *** [absl/flags/CMakeFiles/flags_reflection.dir/build.make:79: absl/flags/CMakeFiles/flags_reflection.dir/reflection.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:2822: absl/flags/CMakeFiles/flags_reflection.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

[paleolimbot]

Thank you!

[paleolimbot]

Also note that on Ubutu 25.10, the system-supplied Abseil-cpp should be new enough (apt-get install libabsl-dev).