um: add asm/percpu.h

To make SLUB work on UML we need this_cpu_cmpxchg from
asm-generic/percpu.h.
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/percpu.h

+#ifndef __UM_PERCPU_H
+#define __UM_PERCPU_H
+
+#include <asm-generic/percpu.h>
+
+#endif /* __UM_PERCPU_H */