libweston: export weston_linux_sync_file_read_timestamp()

This is an internal export for GL-renderer, so that it does not need to build
linux-sync-file.c a second time. This follows the example of
linux-explicit-synchronization.c which is also used by GL-renderer.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>