.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/usb/.gitignore b/tools/usb/.gitignore
index 1b74489..fce1ef5 100644
--- a/tools/usb/.gitignore
+++ b/tools/usb/.gitignore
@@ -1,2 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
 ffs-test
 testusb