fix(nix): add libxkbcommon to buildInputs

[AndrewBastin]

The xkbcommon crate was added to both default features in 0eb4603 (wayland and x11, with default = ["wayland", "x11"]), so every default build now links against the system libxkbcommon. nix/package.nix wasn't updated to match and still only lists wayland and libx11 in buildInputs, so the Nix flake build fails at the link step:

  ld: cannot find -lxkbcommon: No such file or directory
  collect2: error: ld returned 1 exit status
  error: could not compile stochos (bin "stochos") due to 1 previous error

This is the Nix-side equivalent of the CI fix already merged in 82789a2 (fix(ci): install libxkbcommon-dev for xkbcommon linking) - the same missing dependency, just for the Nix build path. The regression first shipped in v0.6.1 and is still present on main.

This PR adds libxkbcommon to the libs list, which covers both buildInputs (fixing the link) and libsPath, so the existing postFixup rpath patch resolves it at runtime too.