tuning: Handle macbeth_ref.pgm license through REUSE.toml

Despite the file starting with a text header, the reuse tool considers
macbeth_ref.pgm as a binary, and doesn't parse it to look for SPDX tags.
Handle it through REUSE.toml instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Acked-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

utils/tuning/libtuning/macbeth_ref.pgm

@@ -1,5 +1,4 @@
 P5
-# SPDX-License-Identifier: BSD-2-Clause
 # Reference macbeth chart
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