.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/samples/bpf/.gitignore b/samples/bpf/.gitignore
index 74d31fd..23837f2 100644
--- a/samples/bpf/.gitignore
+++ b/samples/bpf/.gitignore
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 cpustat
 fds_example
 hbm