spinlock_up.h: include asm/processor.h in for cpu_relax

Commit e66eed651fd1 ("list: remove prefetching from regular list
iterators") removed the include of prefetch.h from list.h and this was a
path to including asm/processor.h.  We need to include it excplicitly
now.

Fixes this build error on sparc32 (at least):

  In file included from include/linux/seqlock.h:29,
                   from include/linux/time.h:8,
                   from include/linux/timex.h:56,
                   from include/linux/sched.h:57,
                   from arch/sparc/kernel/asm-offsets.c:13:
  include/linux/spinlock.h: In function 'spin_unlock_wait':
  include/linux/spinlock.h:360: error: implicit declaration of function 'cpu_relax

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>