staging/android: make sync_timeline internal to sw_sync

staging/android: make sync_timeline internal to sw_sync

The only use sync_timeline will have in upstream kernel is for debugging
through the SW_SYNC interface. So make it internal to SW_SYNC to avoid
people use it in the future.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>