Add missing stdint includes

[onitake]

Arcus uses C stdint types in several places, but the source code is missing stdint.h/cstdint includes. This causes build errors when they aren't pulled in by some other dependency.

Context: This started appearing when the Debian package (an older version, but still affects the current release) was test-built with gcc 15: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1097162

I added a #include <cstdint> in all source files where [u]int[nn]_t types are used.