add missing include for clang12
Fixes the clang12 compilation error:
../Phys/FlavourTagging/src/Taggers/InclusiveTagger/Classifiers/DeepSet/DeepSet_BdBsBu_v031121.h:37:25: error: implicit instantiation of undefined template 'std::basic_ifstream<char>'
std::ifstream f{filename};