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>

drivers/staging/lustre/lustre/llite/dir.c

@@ -45,6 +45,7 @@
 #include <asm/uaccess.h>
 #include <linux/buffer_head.h>   // for wait_on_buffer
 #include <linux/pagevec.h>
+#include <linux/prefetch.h>
 
 #define DEBUG_SUBSYSTEM S_LLITE