src: Use 'include <string.h>' when appropriate

Drop 'include <string.h>' when it is not used and
add it when it is missing.
Also extra lines removed, or added just before local includes.

Change-Id: Iccac4dbaa2dd4144fc347af36ecfc9747da3de20
Signed-off-by: Elyes HAOUAS <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31966
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/drivers/net/ne2k.c

@@ -35,7 +35,6 @@ SMC8416 PIO support added by Andrew Bettison (andrewb@zip.com.au) on 4/3/02
 #include <device/pci_ids.h>
 #include <device/pci_ops.h>
 #include <stdlib.h>
-#include <string.h>
 #include <ip_checksum.h>
 
 #include "ns8390.h"