sandbox: Add asm/errno.h

This file is required for all archs. Fixes a sandbox build break on ext4.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/errno.h b/arch/sandbox/include/asm/errno.h
new file mode 100644
index 0000000..4c82b50
--- /dev/null
+++ b/arch/sandbox/include/asm/errno.h
@@ -0,0 +1 @@
+#include <asm-generic/errno.h>