string-machine: fix build against `gcc-13`
Without the change the build fails as
https://hydra.nixos.org/build/246390341:
sources/ui/Color.h:4:5: error: 'uint8_t' does not name a type
4 | uint8_t r, g, b, a;
| ^~~~~~~
sources/ui/Color.h:1:1: note: 'uint8_t' is defined in header '<cstdint>'; did you forget to '#include <cstdint>'?
+++ |+#include <cstdint>
1 | #pragma once