docs: Do not include from .../seqno-fence.c

There are no kerneldoc comments in drivers/dma-buf/seqno-fence.c, and it
appears there never have been.  Stop looking for comments there to
eliminate this warning:

    ./drivers/dma-buf/seqno-fence.c:1: warning: no structured comments found

Signed-off-by: Jonathan Corbet <corbet@lwn.net>