staging: wilc1000: fix build error for openrisc-allmodconfig

The kbuild test robot reports the following build error for
openrisc-allmodconfig.

>> drivers/staging/wilc1000/wilc_sdio.c:584:2: error: implicit
>> declaration of function 'memset'

The error occurs due to missing a standard header file as
<linux/string.h> so that three .c files are included it.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wilc1000/wilc_sdio.c

@@ -7,6 +7,7 @@
 /*  */
 /* //////////////////////////////////////////////////////////////////////////// */
 
+#include <linux/string.h>
 #include "wilc_wlan_if.h"
 #include "wilc_wlan.h"
 

drivers/staging/wilc1000/wilc_spi.c

@@ -7,6 +7,7 @@
 /*  */
 /* //////////////////////////////////////////////////////////////////////////// */
 
+#include <linux/string.h>
 #include "wilc_wlan_if.h"
 #include "wilc_wlan.h"
 

drivers/staging/wilc1000/wilc_wlan_cfg.c

@@ -7,6 +7,7 @@
 /*  */
 /* ///////////////////////////////////////////////////////////////////////// */
 
+#include <linux/string.h>
 #include "wilc_wlan_if.h"
 #include "wilc_wlan.h"
 #include "wilc_wlan_cfg.h"