CountersUnitTest: Fix C++14 compiler warning

C++14 requires a message for static_assert: !786 (comment 1916629)

mentioned in merge request !786 (merged)

changed title from Fix C++14 compiler warning to CountersUnitTest: Fix C++14 compiler warning

approved this merge request

assigned to @clemenci

changed milestone to %v30r5

enabled an automatic merge when the pipeline for 91ed4a89 succeeds

• [2018-11-05 10:27] Validation started with lhcb-gaudi-merge#629

mentioned in commit c1fec244

merged