extconf.rb: libffi with libruby

* ext/fiddle/extconf.rb: link libffi with libruby for symbols
  provided by ruby on some platforms, e.g. alloca.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48919 b2dd03c8-39d4-4d8f-98ff-823fe69b080e