fs: Disable sandbox filesystem in SPL

Don't bother compiling the sandbox filesystem in SPL for now, as it is not
needed.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/fs/fs.c b/fs/fs.c
index cfc781b..4cb4310 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -237,7 +237,7 @@
 		.mkdir = fs_mkdir_unsupported,
 	},
 #endif
-#ifdef CONFIG_SANDBOX
+#if IS_ENABLED(CONFIG_SANDBOX) && !IS_ENABLED(CONFIG_SPL_BUILD)
 	{
 		.fstype = FS_TYPE_SANDBOX,
 		.name = "sandbox",