libskk: fix cross build

This ensures the proper configureFlags are passed to autogen.sh

pkgs/by-name/li/libskk/package.nix

···
     json-glib
   ];
 
-  preConfigure = ''
-    ./autogen.sh
-  '';
+  configureScript = "./autogen.sh";
 
   # link SKK-JISYO.L from skkdicts for the bundled tool `skk`
   preInstall = ''