Bug#974020: std::set used in non-thread-safe way

[Michal Nowak]

Package: softhsm
Version: 2.4.0-0.1

We see (https://gitlab.isc.org/isc-projects/bind9/-/issues/2253) the 
following bug in Buster's SoftHSM from time to time when running BIND9 
system tests: https://github.com/opendnssec/SoftHSMv2/issues/518.

This has been fixed upstream in SoftHSM 2.6.0 via 
https://github.com/opendnssec/SoftHSMv2/pull/519 but not backported as 
SoftHSM 2.4 is discontinued upstream.