sandbox: Add some missing headers in cpu.c

These headers are needed in case they are not transitively included.

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

arch/sandbox/cpu/cpu.c

@@ -4,6 +4,8 @@
  */
 #define DEBUG
 #include <common.h>
+#include <errno.h>
+#include <libfdt.h>
 #include <os.h>
 #include <asm/io.h>
 #include <asm/state.h>