prelude.c.tmpl: strip prefix

* template/prelude.c.tmpl (Prelude#translate): strip VPATH prefix
  from prelude names, so that srcdir diffrences do not make the
  generated code diffrent.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48490 b2dd03c8-39d4-4d8f-98ff-823fe69b080e