staging/lustre/llite: add missing include file for prefetchw

Got below errors on s390 build:
  CC [M]  drivers/staging/lustre/lustre/llite/dir.o
drivers/staging/lustre/lustre/llite/dir.c: In function 'll_dir_filler':
drivers/staging/lustre/lustre/llite/dir.c:225:3: error: implicit declaration of function 'prefetchw' [-Werror=implicit-function-declaration]

Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>