Install libffi-dev in SAIO docs.

If you don't, then newer versions of xattr won't install, and since
our xattr requirement is simply ">= 0.4" in requirements.txt, this
affects anyone setting up a new SAIO.

This happened with xattr 0.7, which was released on 2013-07-19.

Change-Id: Iaf335fa25a2908953d1fd218158ebedf5d01cc27
Merge "Install libffi-dev in SAIO docs."