xhtml1: set license (#517233)

lib/licenses/licenses.nix

@@ -1560,6 +1560,11 @@ lib.mapAttrs mkLicense (
       fullName = "W3C Software Notice and License";
     };
 
+    w3c-19980720 = {
+      spdxId = "W3C-19980720";
+      fullName = "W3C Software Notice and License (1998-07-20)";
+    };
+
     wadalab = {
       fullName = "Wadalab Font License";
       url = "https://fedoraproject.org/wiki/Licensing:Wadalab?rd=Licensing/Wadalab";

pkgs/by-name/xh/xhtml1/package.nix

@@ -33,5 +33,6 @@ stdenv.mkDerivation {
     homepage = "https://www.w3.org/TR/xhtml1/";
     description = "DTDs for XHTML 1.0, the Extensible HyperText Markup Language";
     platforms = lib.platforms.unix;
+    license = lib.licenses.w3c-19980720;
   };
 }