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>

lib/fdtdec_test.c

@@ -79,7 +79,9 @@ static int make_fdt(void *fdt, int size, const char *aliases,
 {
 	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));