create-spdx: cross recipes are native also

Recipes that inherit cross should also be categorized as isNative

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit ee113e38)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>