.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/.gitignore b/.gitignore
index 72ef86a..2258e90 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 #
 # NOTE! Don't add files that are generated in specific
 # subdirectories here. Add them in the ".gitignore" file