libweston: Use struct timespec for button events

libweston: Use struct timespec for button events

Change code related to button events to use struct timespec to represent
time.

This commit is part of a larger effort to transition the Weston codebase
to struct timespec.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>