setup.py: ship the clang plugin

Without this, installed packages don't work very well for indexing.

Note that I don't know my way around setuptools/distribute/whatever so don't trust me on this - it's more of a bug report that _not_ doing this doesn't work, I guess?  Anyway, with `setup.py install`, it didn't include the .so so actually using clang wasn't very productive.  If we're going to ship the rest of the clang plugin anyway...