irssi: add license

see issue #43716

pkgs/applications/networking/irc/irssi/default.nix

···
     description = "A terminal based IRC client";
     platforms   = stdenv.lib.platforms.unix;
     maintainers = with stdenv.lib.maintainers; [ lovek323 ];
+    license     = stdenv.lib.licenses.gpl2Plus;
   };
 }