sandbox: Add missing errno.h includes in a few files

These files use error numbering, so add the include.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index b23d08b..4c38fab 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <errno.h>
 #include <os.h>
 #include <cli.h>
 #include <malloc.h>