staging/otus: include slab.h from usbdrv.h

drivers/staging/otus/usbdrv.h users use slab facilities.  Include
linux/slab.h from usbdrv.h.

Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/drivers/staging/otus/usbdrv.h b/drivers/staging/otus/usbdrv.h
index 330d1b9..7e66c2d 100644
--- a/drivers/staging/otus/usbdrv.h
+++ b/drivers/staging/otus/usbdrv.h
@@ -38,6 +38,7 @@
 #include <linux/uaccess.h>
 #include <linux/wireless.h>
 #include <linux/if_arp.h>
+#include <linux/slab.h>
 #include <linux/io.h>
 
 #include "zdcompat.h"