um: include linux/prefetch.h

Fix build failures on UML.

Signed-off-by: Richard Weinberger <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

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

···
 #include "registers.h"
 #include "sysdep/archsetjmp.h"
 
+#include <linux/prefetch.h>
+
 struct mm_struct;
 
 struct thread_struct {