i40evf: fix s390 build failure due to implicit prefetch.h

As of commit 7f12ad7 ("i40evf: transmit
and receive functionality") the s390 builds (allyesconfig) fail with:

drivers/net/ethernet/intel/i40evf/i40e_txrx.c: In function 'i40e_clean_rx_irq':
drivers/net/ethernet/intel/i40evf/i40e_txrx.c:818:3: error: implicit declaration of function 'prefetch'
make[5]: *** [drivers/net/ethernet/intel/i40evf/i40e_txrx.o] Error 1

due to an implicit assumption that the prototype from linux/prefetch.h
will be present.

Cc: Mitch Williams <mitch.a.williams@intel.com>
Cc: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40evf/i40e_txrx.c

@@ -21,6 +21,8 @@
  *
  ******************************************************************************/
 
+#include <linux/prefetch.h>
+
 #include "i40evf.h"
 
 static inline __le64 build_ctob(u32 td_cmd, u32 td_offset, unsigned int size,