Fix build with GCC 13

[hlounent]

The header uses fixed-width integer types such as uint32_t but does not include .

The build failed with

[8/11] Building CXX object example/traits/CMakeFiles/boost-json.dir/boost-json.cpp.o
FAILED: example/traits/CMakeFiles/boost-json.dir/boost-json.cpp.o
/usr/bin/c++ -DBOOST_JSON_STANDALONE -I/path/to/src/jwt_cpp/include  -MD -MT example/traits/CMakeFiles/boost-json.dir/boost-json.cpp.o -MF example/traits/CMakeFiles/boost-json.dir/boost-json.cpp.o.d -o example/traits/CMakeFiles/boost-json.dir/boost-json.cpp.o -c /path/to/src/jwt_cpp/example/traits/boost-json.cpp
In file included from /path/to/src/jwt_cpp/include/jwt-cpp/jwt.h:12,
                    from /path/to/src/jwt_cpp/include/jwt-cpp/traits/boost-json/traits.h:5,
                    from /path/to/src/jwt_cpp/example/traits/boost-json.cpp:1:
/path/to/src/jwt_cpp/include/jwt-cpp/base.h:91:24: error: ‘uint32_t’ does not name a type
    91 |                 inline uint32_t index(const std::array<char, 64>& alphabet, char symbol) {
        |                        ^~~~~~~~
/path/to/src/jwt_cpp/include/jwt-cpp/base.h:9:1: note: ‘uint32_t’ is defined in header ‘<cstdint>’; did you forget to ‘#include <cstdint>’?
    8 | #include <vector>
    +++ |+#include <cstdint>
    9 |

with

gcc (GCC) 13.0.1 20230401 (Red Hat 13.0.1-0)
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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