shadow: Add missing SPDX headers

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-oniro-staging/recipes-extended/shadow/shadow_%.bbappend

+# SPDX-FileCopyrightText: Huawei Inc.
+#
+# SPDX-License-Identifier: MIT
+
 do_install:append () {
 	# to hardend
 	sed -i -e 's:UMASK.*:UMASK 027:' ${D}${sysconfdir}/login.defs