libweston: Add check and fallback for linux/sync_file.h

libweston: Add check and fallback for linux/sync_file.h

The sync file functionality is required by the upcoming GPU render
timeline work, but it's only available in relatively new linux kernel
versions (4.7 and above).

This commit provides a "sanitized" version of the required sync file
definitions.  On systems that don't have the sync file header (due to
having an older kernel), we will be able to fall back to our own
definitions when building.

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