Include cstdint for compatibility with GCC 15

[pastalian]

cstdint needs to be included explicitly since GCC 15.
https://gcc.gnu.org/gcc-15/porting_to.html

Downstream bug: https://bugs.gentoo.org/937500

$ cmake -B build -DUSE_WS=ON -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF
$ cmake --build build
[ 93%] Building CXX object ws/CMakeFiles/ws.dir/__/third_party/msgpack11/msgpack11.cpp.o
In file included from /IXWebSocket/third_party/msgpack11/msgpack11.cpp:1:
/IXWebSocket/third_party/msgpack11/msgpack11.hpp:53:25: error: 'uint8_t' was not declared in this scope
   53 |     typedef std::vector<uint8_t> binary;
      |                         ^~~~~~~

[bsergean]

thanks !