[Bug #19550]

If !RCLASS_EXT_EMBEDDED (e.g. 32 bit systems) then the rb_classext_t is
allocated throug malloc so it must be freed.

The issue can be seen in the following script:

```
20.times do
  100_000.times do
    mod = Module.new
    Class.new do
      include mod
    end
  end

  # Output the Resident Set Size (memory usage, in KB) of the current Ruby process
  puts `ps -o rss= -p #{$$}`
end
```

Before this fix, the max RSS is 280MB, while after this change, it's
30MB.