BoostedJetTaggers macOS / Clang Fix, 21.2 branch (2020.03.20.)

While trying to build AnalysisBase-21.2.114 on macOS Catalina, I ran into an issue in compiling this class.

It seems that libstdc++ automatically includes the definition of std::string when one includes <map>. But libc++ doesn't.

In any case, the update should really not be controversial...