Error uninitialized constant Truffle::FFI::Library

truffleruby 20.2.0, like ruby 2.6.6, GraalVM CE Native [x86_64-darwin]

Just testing one of my ffi libs with TruffleRuby and I hit this:

```
require 'ffi'

class File::Temp < File
  extend FFI::Library # boom
end
```

Result:

```
test.rb:4:in `const_missing': uninitialized constant Truffle::FFI::Library (NameError)
	from test.rb:4:in `Temp'
	from test.rb:3:in `<main>'
```