book: Fix sys crate import to use the 'ffi' convention.
book: Fix sys crate import to use the 'ffi' convention.

Gir now generates code expecting the crate to be imported as 'ffi'.

GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir,gtk-rs/gir