weston-info: destroy wl_keyboard

Fixes a memory leak by calling wl_keyboard_destroy on any keyboard
that was used to listen for events.

Signed-off-by: Markus Ongyerth <wl@ongy.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>