d/a/copyright: Whitelist GPL-3 licensed files

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

debian/apertis/copyright.whitelist

+# GPL-3+ test. Not distributed.
+libio/tst-fwrite-error.c
+# GPL-3+ build script. Doesn't affect the artifacts.
+scripts/move-if-change
+# LGPL-3+ Hurd Thread Library. Ignore as Hurd is not targetted.
+sysdeps/htl/pt-kill.c
+sysdeps/htl/pt-mutex-transfer-np.c
+sysdeps/htl/raise.c