rc.conf: Leap-seconds sources: Add IANA back

11da791920ba switched Leap-seconds source from IANA to IERS, as IERS
is the canonoical source. The problem ist that IERS is not accessible
from IPv6 only networks. To work around this we must add IANA back in
order to provide IPv6-only users a source for leap-seconds fetch.

PR: 279413
Fixes: 11da791920ba
MFC after: 3 days