gnome2.libglademm: split out devdoc output

pkgs/desktops/gnome-2/bindings/libglademm/default.nix

···
     url = "mirror://gnome/sources/libglademm/2.6/${name}.tar.bz2";
     sha256 = "1hrbg9l5qb7w0xvr7013qamkckyj0fqc426c851l69zpmhakqm1q";
   };
-  
+
+  outputs = [ "out" "devdoc" ];
+
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ intltool ];