net: fec: fix build error at m68k platform

reproduce:
  wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
  chmod +x ~/bin/make.cross
  git checkout 1b7bde6d659d30f171259cc2dfba8e5dab34e735

  make.cross ARCH=m68k m5275evb_defconfig
  make.cross ARCH=m68k

All error/warnings:

   drivers/net/ethernet/freescale/fec_main.c: In function 'fec_enet_rx_queue':
>> drivers/net/ethernet/freescale/fec_main.c:1470:3: error: implicit declaration of function 'prefetch' [-Werror=implicit-function-declaration]
      prefetch(skb->data - NET_IP_ALIGN);
      ^
   cc1: some warnings being treated as errors

missed included prefetch.h

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_main.c

@@ -57,6 +57,7 @@
 #include <linux/regulator/consumer.h>
 #include <linux/if_vlan.h>
 #include <linux/pinctrl/consumer.h>
+#include <linux/prefetch.h>
 
 #include <asm/cacheflush.h>