Remove native binaries from main jar

When building jruby-complete, I noticed that it has transitive
dependencies on both the "main" jffi jar, and the "native" jffi
jar.  Prior to this commit, both jars contain a copy of the native
binaries, so one set is getting overwritten by the other set
during the construction of the final jruby-complete jar via
the shade plugin.

Raised the question as to whether this was intentional or not
in IRC #jruby, and it sounded like it was not.  This commit
would change the packaging so that the native binaries are only
packaged in the native jar, and give end users a bit more control
over what to include/exclude in downstream projects.