btrfs: add SPDX header to Kconfig

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/Kconfig

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0
+
 config BTRFS_FS
 	tristate "Btrfs filesystem support"
 	select LIBCRC32C