Add missing std::variant include

Following up on gaudi/Gaudi#238 (closed) it turned out that variant was not properly included (probably because implied by the include of boost::hash).

This MR adds the missing includes so that when I address gaudi/Gaudi#238 (closed) the code will still compile.

added RTA label

assigned to @msaur

assigned to @clemenci

/ci-test --platforms==x86_64_v2-centos7-gcc11-opt

added ci-test-triggered label

• [2022-09-09 11:47] Validation started with lhcb-master-mr#5608
• [2022-09-09 12:03] Validation started with lhcb-master-mr#5609

resolved all threads

added cleanup label

approved this merge request

merged

mentioned in commit 778c691d

mentioned in issue Moore#478 (closed)