um: fix build without CONFIG_UML_TIME_TRAVEL_SUPPORT

When CONFIG_UML_TIME_TRAVEL_SUPPORT isn't set, the build was broken.
Fix this.

Fixes: 06503870

 ("um: Support time travel mode")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>