libtuning: Fix reference image

Opencv fails to load the image. The added license destroys the magic
number. Fix, by moving the licence below the magic number.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Paul ELder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

utils/tuning/libtuning/macbeth_ref.pgm

@@ -1,5 +1,5 @@
-# SPDX-License-Identifier: BSD-2-Clause
 P5
+# SPDX-License-Identifier: BSD-2-Clause
 # Reference macbeth chart
 120 80
 255