.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/riscv/kernel/.gitignore b/arch/riscv/kernel/.gitignore
index b51634f..e052ed33 100644
--- a/arch/riscv/kernel/.gitignore
+++ b/arch/riscv/kernel/.gitignore
@@ -1 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
 /vmlinux.lds