udlfb: include prefetch.h explicitly

Commit e66eed65 ("list: remove prefetching from
regular list iterators") removed the include of prefetch.h from list.h,
so we need to include it explicitly, now.

fixes this build error on powerpc:

drivers/video/udlfb.c: In function 'dlfb_compress_hline':
drivers/video/udlfb.c:421: error: implicit declaration of function 'prefetch_range'

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>