fdt: sandbox: Avoid looking for an appended device tree

We don't use an appended tree for sandbox and the required symbols are
not present. Add a condition to avoid a build error.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/fdtdec.c

@@ -1225,6 +1225,9 @@ static void *fdt_find_separate(void)
 {
 	void *fdt_blob = NULL;
 
+	if (IS_ENABLED(CONFIG_SANDBOX))
+		return NULL;
+
 #ifdef CONFIG_SPL_BUILD
 	/* FDT is at end of BSS unless it is in a different memory region */
 	if (CONFIG_IS_ENABLED(SEPARATE_BSS))