xvkbd: 3.8 -> 3.9

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 3.9 with grep in /nix/store/254qz2nmlqkyzn1hv598i5y9spx50wji-xvkbd-3.9
- found 3.9 in filename of file in /nix/store/254qz2nmlqkyzn1hv598i5y9spx50wji-xvkbd-3.9