camomile.0.8.5: decouple from camlp4

Arguably opam-repository is not the place to add such patches, but:
  * Camomile is not very actively developed
  * The patch is tiny and safe; conceptually it's just one invocation of `camlp4o`
  * I would really like to have a basic camlp4-free system, which means utop, which means camomile
  * opam-repository already includes a similar patch for https://github.com/yoriyuki/Camomile/pull/7 / https://github.com/yoriyuki/Camomile/pull/8.

This has been also reported upstream: https://github.com/yoriyuki/Camomile/issues/10.