xfs: xfs_rtget_summary can be static

Fix sparse warning introduced by commit afabfd30 ("xfs: combine
xfs_rtmodify_summary and xfs_rtget_summary").

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>