GaudiTestSuite: fix missing definition of static member

After !1696 (merged) I can no longer build GaudiTestSuite locally due to

ERROR: failed to load libGaudiTestSuiteComponents.so: /scratch/fwinkl/Gaudi/build/.plugins/libGaudiTestSuiteComponents.so: undefined symbol: _ZN18ExtendedProperties6hasherB5cxx11E

No idea why this works in the CI.