ikiwiki: add highlight dependency
ikiwiki: add highlight dependency

IkiWiki's highlight plugin requires this.

Note that the current highlight version (3.57) is not compatible with
ikiwiki version in nixpkgs. This will be fixed in the next commit.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs