slub: include include for prefetch

Otherwise m68k breaks:

On Mon, 30 Jan 2012, Geert Uytterhoeven wrote:
> m68k/allmodconfig at http://kisskb.ellerman.id.au/kisskb/buildresult/5527349/
>
> mm/slub.c:274: error: implicit declaration of function 'prefetch'
>
> Sorry, didn't notice it earlier due to other build breakage in -next.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -29,6 +29,7 @@
 #include <linux/math64.h>
 #include <linux/fault-inject.h>
 #include <linux/stacktrace.h>
+#include <linux/prefetch.h>
 
 #include <trace/events/kmem.h>