.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/tools/spi/.gitignore b/tools/spi/.gitignore
index 4280576..14ddba3 100644
--- a/tools/spi/.gitignore
+++ b/tools/spi/.gitignore
@@ -1,2 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
 spidev_fdx
 spidev_test