Fixes dependency
* change dependency on py-cairo and gobject-introspection to build.
  Those are required by configure script, but not required by
  installed files. Furthermore, this package is python-version-independent
  part of py-gobject, but full dependency of those packages introduce
  dependency on certain python version.
* add direct full dependency on libffi, because it is already in  buildlink3.mk.
  (previously, it was indirect dependency from gobject-introspection, but
  py-gobject is using it directly).

Bump PKGREVISION.

(obache)