staging: lustre: fix build error in mdc_request.c

Turns out we need some more .h files to build properly on all arches.
Specifically prefetch.h for this file.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 73d65c8d1a85 ("staging: lustre: remove libcfs_all.h from lustre/include/*.h")
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/mdc/mdc_request.c

@@ -40,6 +40,7 @@
 # include <linux/utsname.h>
 # include <linux/file.h>
 # include <linux/kthread.h>
+#include <linux/prefetch.h>
 
 #include <lustre_errno.h>
 #include <cl_object.h>