.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/arch/um/.gitignore b/arch/um/.gitignore
index a73d3a1..6323e55 100644
--- a/arch/um/.gitignore
+++ b/arch/um/.gitignore
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0-only
 kernel/config.c
 kernel/config.tmp
 kernel/vmlinux.lds