um: asm-offsets.h might as well come from underlying arch...

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/include/asm/asm-offsets.h

-#include <generated/asm-offsets.h>