fdtdec: test: Fix build warning

Hide the declaration of the "fd" variable When not building a DEBUG
configuration, to avoid the variable being unused.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec_test.c b/lib/fdtdec_test.c
index a82e27d..065fed2 100644
--- a/lib/fdtdec_test.c
+++ b/lib/fdtdec_test.c
@@ -79,7 +79,9 @@
 {
 	char name[20], value[20];
 	const char *s;
+#if defined(DEBUG) && defined(CONFIG_SANDBOX)
 	int fd;
+#endif
 
 	CHECK(fdt_create(fdt, size));
 	CHECK(fdt_finish_reservemap(fdt));