Fix build with recent Boost

It has static_assert complaining on missing include

Global.h

@@ -150,6 +150,7 @@ static_assert(sizeof(bool) == 1, "Bool needs to be 1 byte in size.");
 #endif
 #include <boost/logic/tribool.hpp>
 #include <boost/optional.hpp>
+#include <boost/optional/optional_io.hpp>
 #include <boost/program_options.hpp>
 #include <boost/range/adaptor/filtered.hpp>
 #include <boost/range/adaptor/reversed.hpp>