Fix class/struct mismatch warning from clang

this fixes the warning:

In file included from ../GaudiExamples/src/FunctionalAlgorithms/MakeAndConsume.cpp:12:
../GaudiAlg/include/GaudiAlg/FunctionalTool.h:25:5: warning: 'ToolBinder' defined as a class template here but previously declared as a struct template; this is valid, but may result in linker errors under the Microsoft C++ ABI [-Wmismatched-tags]
    class ToolBinder<Gaudi::Interface::Bind::Box<IFace>( Args const&... ), Traits>
    ^
../GaudiAlg/include/GaudiAlg/FunctionalTool.h:22:5: note: did you mean class here?
    struct ToolBinder;
    ^~~~~~
    class

/cc @graven