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

GaudiAlg/include/GaudiAlg/FunctionalTool.h

@@ -19,7 +19,7 @@ namespace Gaudi::Functional {
                                           Traits::use_<Tr, BaseClass_t<Base>>>;
 
     template <typename Signature, typename Traits>
-    struct ToolBinder;
+    class ToolBinder;
 
     template <typename IFace, typename... Args, typename Traits>
     class ToolBinder<Gaudi::Interface::Bind::Box<IFace>( Args const&... ), Traits>