usbguard-notifier: fix build against `gcc-13`

Without the change `usbguard-notifier` fails the build on `staging-next`
as:

    In file included from src/Notifier.hpp:24,
                     from src/Main.cpp:21:
    src/Serializer.hpp:35:5: error: 'uint32_t' does not name a type
       35 |     uint32_t device_id;
          |     ^~~~~~~~
    src/Serializer.hpp:25:1: note: 'uint32_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
       24 | #include <map>
      +++ |+#include <cstdint>
       25 | #include <string>

0 files changed, 0 insertions, 0 deletions