xmldiff 0.3.0 and 0.4.0 depend on camlp4

xmldiff depends on camlp4, as [shown in the bulk logs](https://github.com/ocaml/opam-bulk-logs/blob/master/ubuntu-trusty/4.02.0/20140930/raw/xmldiff#L60-L62):

```
checking for camlp4... no
Exception: Program_not_found "camlp4".
[ERROR] The compilation of xmldiff.0.3.0 failed.
```