input: don't send to clients key events eaten by bindings

weston key bindings are supposed to eat the key events, and not pass it
on to clients, and indeed the wl_keyboard.key event is not sent. But
we must also not put the key in the keys array to pass to client with
the wl_keyboard.enter event, or else we may send the 'eaten' one too.
In the case of a key binding hiding a surface having the keyboard focus,
the shell may decide to give the focus to another surface, but that will
happen before the key is released, so the new focus surface will receive
the code of the bound key in the wl_keyboard.enter array.
headless: make it build again

This doesn't make the headless backend libweston compatible, it
just make it build.