The upgrade to GCC 15 broke the build because `cstdint` isn't getting
transitively included via some other headers anymore [1].
This has been fixed in newer 1.x versions of synergy, but those include
some more drastic changes that would need to be addressed in Nixpkgs
(such as, at least, in the NixOS and nix-darwin synergy modules).
I've opened a PR for the update here [2], but this patch is a simpler
quick fix we can implement before that's sorted out.
[1]: https://gcc.gnu.org/gcc-15/porting_to.html#header-dep-changes
[2]: https://github.com/NixOS/nixpkgs/pull/486784
