um: rtc: Avoid shadowing err in uml_rtc_start()

Remove the declaration of 'err' inside the 'if (timetravel)' block,
as it would otherwise be unavailable outside that block, potentially
leading to uml_rtc_start() returning an uninitialized value.

Fixes: dde8b58d ("um: add a pseudo RTC")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250708090403.1067440-5-tiwei.bie@linux.dev


Signed-off-by: Johannes Berg <johannes.berg@intel.com>