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: 73d65c8d ("staging: lustre: remove libcfs_all.h from lustre/include/*.h")
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>