xorg.libXfixes: 5.0.2 -> 5.0.3

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 5.0.3 with grep in /nix/store/g5hcg35wmg25sgfjp7mvi4cx3shldbxd-libXfixes-5.0.3
- directory tree listing: https://gist.github.com/7398ada0908969ebbd1e7e629a1e0ef7