unicode_norm_gen.tmpl: fix build error

* template/unicode_norm_gen.tmpl: fix build error on CentOS 5 with
  ruby 1.8.5 which lacks Symbol#to_proc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48377 b2dd03c8-39d4-4d8f-98ff-823fe69b080e