sparc32: fix build, fix missing cpu_relax declaration

Fix following sparc (32 bit) build error:

  CC      arch/sparc/kernel/asm-offsets.s
In file included from include/linux/seqlock.h:29:0,
                 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:360:2: error: implicit declaration of function 'cpu_relax'

Most likely caused by commit e66eed65 ("list: remove
prefetching from regular list iterators") due to include
changes.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>