um: Fix xor.h include

Two independent changes here ended up going into the tree
one after another, without a necessary rename, fix that.

Reported-by: Thomas Meyer <thomas@m3y3r.de>
Fixes: f185063b

 ("um: Move timer-internal.h to non-shared")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>