.gitignore: add SPDX License Identifier

Add SPDX License Identifier to all .gitignore files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/security/selinux/.gitignore b/security/selinux/.gitignore
index 2e5040a..168fae1 100644
--- a/security/selinux/.gitignore
+++ b/security/selinux/.gitignore
@@ -1,2 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
 av_permissions.h
 flask.h