libiconv: expose setup hooks for use by other libiconv implementations

pkgs/development/libraries/libiconv/default.nix

···
     (lib.enableFeature enableShared "shared")
   ] ++ lib.optional stdenv.isFreeBSD "--with-pic";
 
+  passthru = { inherit setupHooks; };
+
   meta = {
     description = "An iconv(3) implementation";