Revert "libical: temporarily don't build docs to unblock staging-next"

This reverts commit dae72e7.
Not needed now.

pkgs/development/libraries/libical/default.nix

@@ -5,7 +5,7 @@ stdenv.mkDerivation rec {
   name = "libical-${version}";
   version = "3.0.4";
 
-  outputs = [ "out" "dev" ]; #"devdoc" ];
+  outputs = [ "out" "dev" "devdoc" ];
 
   src = fetchFromGitHub {
     owner = "libical";
@@ -17,8 +17,7 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [
     perl pkgconfig cmake ninja vala gobject-introspection
     (python3.withPackages (pkgs: with pkgs; [ pygobject3 ])) # running libical-glib tests
-# Docs building fails: https://github.com/NixOS/nixpkgs/pull/61657#issuecomment-495579489
-#    gtk-doc docbook_xsl docbook_xml_dtd_43 # docs
+    gtk-doc docbook_xsl docbook_xml_dtd_43 # docs
   ];
   buildInputs = [ glib libxml2 icu ];