Hadrian: Add Missing Libffi Dependencies

There are a handful of fsatrace linting errors (missing needs) when building libffi. Libffi is ultimately built from a single tar.gz file (e.g. libffi-tarballs/libffi-3.99999+git20171002+77e130c.tar.gz). File can be seen as the shallow (file) dependency for the whole libffi build. Hence it should be sufficient to need that file in all libffi rules, and trackAllow the whole libffi build directory.