xtensa: fix a build error in arch/xtensa/include/asm/uaccess.h

Fix the following build error:

  arch/xtensa/include/asm/uaccess.h:403: error: implicit declaration of function 'prefetch'
  arch/xtensa/include/asm/uaccess.h:412: error: implicit declaration of function 'prefetchw'

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Chris Zankel <chris@zankel.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>