libweston: Use struct timespec for compositor time

libweston: Use struct timespec for compositor time

Change weston_compositor_get_time to return the current compositor time
as a struct timespec. Also, use clock_gettime (with CLOCK_REALTIME) to
get the time, since it's equivalent to the currently used gettimeofday
call, but returns the data directly in a struct timespec.

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>