racket-mode: Set files to *.el

Although I've added just one additional new file,
`racket-keywords-and-builtins.el`, I'm too lazy to type that here.

Seriously: I reckon specifying *.el is "future proof", in the same way
it was recommended to just specify *.rkt. However if that's not good
practice, let me know and I'll update the PR to enumerate the *.el files
instead.