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: NTejun Heo <tj@kernel.org>

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"