sandbox: Add the handoff header for spl

This defines a function declared in handoff.h so add the header.
Signed-off-by: NSimon Glass <sjg@chromium.org>

arch/sandbox/cpu/spl.c

@@ -6,6 +6,7 @@
 #include <common.h>
 #include <dm.h>
 #include <hang.h>
+#include <handoff.h>
 #include <init.h>
 #include <log.h>
 #include <os.h>