xfs: advertise realtime quota support in the xqm stat files

Add a fifth column to this (really old) stat file to advertise that the
kernel supports quota for realtime volumes.  This will be used by
fstests to detect kernel support.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>