.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/iio/.gitignore b/tools/iio/.gitignore
index 3758202..5bd6f4d 100644
--- a/tools/iio/.gitignore
+++ b/tools/iio/.gitignore
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 iio_event_monitor
 iio_generic_buffer
 lsiio