package/supervisor: fix license

package/supervisor: fix license

trackrefs module has been removed since version 4.0.0 and
https://github.com/Supervisor/supervisor/commit/6c1c0adadaf190e77dc017a8a40c90249f0a055b

Since this removal, supervisor does not contain any ZPL-2.1 code
anymore, see
https://github.com/Supervisor/supervisor/commit/abff4cd872d2cbd1b999c1f439fdf83b4695cc6c

Fixes:
 - http://autobuild.buildroot.org/results/540303f884d9f891f58c660da22e2273956a92a6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>