errno: Add copyright header and header guard

Bring in a copyright for this file from cmd/pmic.c since this file was
submitted by the same author at around the same time. Also fix the missing
header guard.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>

include/errno.h

+/*
+ * Copyright (C) 2014 Samsung Electronics
+ * Przemyslaw Marczak <p.marczak@samsung.com>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
 #ifndef _ERRNO_H
+#define _ERRNO_H
 
 #include <asm-generic/errno.h>