libffi is generally present as it's also a build dependency on glib-2.0, but
explicitly declare it for determinism.
(From OE-Core rev: 85232b154dbaf3fc2ed3fa9291e3cbeaa7f318ab)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>