wcslib: 5.15 -> 5.18

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 5.18 with grep in /nix/store/pr88acrgc8w11q3p8407w7v0kwd2asq8-wcslib-5.18
- found 5.18 in filename of file in /nix/store/pr88acrgc8w11q3p8407w7v0kwd2asq8-wcslib-5.18