um: include linux/prefetch.h

Fix build failures on UML.
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/include/asm/processor-generic.h

@@ -14,6 +14,8 @@ struct task_struct;
 #include "registers.h"
 #include "sysdep/archsetjmp.h"
 
+#include <linux/prefetch.h>
+
 struct mm_struct;
 
 struct thread_struct {