Add missing include needed when building for centos9
This is meant to fix a problem I found when trying to build on CentOS9:
../Event/EventBase/tests/src/test_SOAPacker.cpp:80:3: required from here
../Event/EventBase/include/Event/SOAPacker.h:52:21: error: 'isnan' was not declared in this scope; did you mean 'std::isnan'?
52 | if ( isnan( dataA[i] ) && isnan( dataB[i] ) ) {
| ~~~~~^~~~~~~~~~~~
| std::isnan