Windows build fails with new JSON conversion

[tony-p]

From my research the failing build is maybe due to MSVC not conforming to the C++ standard

https://www.reddit.com/r/cpp_questions/comments/4agiol/avoiding_ambiguous_symbol_error_in_msvc_gcc/

C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\json_export.h(104,5): error C2872: 'to_json': ambiguous symbol [C:\Users\me\BehaviorTree.CPP\build\examples\t12_groot_howto.vcxproj]
  (compiling source file '../../examples/t12_groot_howto.cpp')
  C:\Users\me\BehaviorTree.CPP\examples\t12_groot_howto.cpp(24,1):
  could be 'void to_json(nlohmann::json_abi_v3_11_2::json &,const Position2D &,bool)'
  C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\contrib\json.hpp(6387,5):
  or       'const T &nlohmann::json_abi_v3_11_2::to_json'
          with
          [
              T=nlohmann::json_abi_v3_11_2::detail::to_json_fn
          ]
  C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\json_export.h(104,5):
  the template instantiation context (the oldest one first) is
        C:\Users\me\BehaviorTree.CPP\examples\t12_groot_howto.cpp(106,7):
        see reference to function template instantiation 'void BT::RegisterJsonDefinition<Position2D>(void)' being compiled
        C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\json_export.h(130,22):
        see reference to function template instantiation 'void BT::JsonExporter::addConverter<T>(void)' being compiled
          with
          [
              T=Position2D
          ]

C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\json_export.h(111,5): error C2872: 'from_json': ambiguous symbol [C:\Users\me\BehaviorTree.CPP\build\examples\t12_groot_howto.vcxproj]
  (compiling source file '../../examples/t12_groot_howto.cpp')
  C:\Users\me\BehaviorTree.CPP\examples\t12_groot_howto.cpp(24,1):
  could be 'void from_json(const nlohmann::json_abi_v3_11_2::json &,Position2D &)'
  C:\Users\me\BehaviorTree.CPP\include\behaviortree_cpp\contrib\json.hpp(5642,5):
  or       'const T &nlohmann::json_abi_v3_11_2::from_json'
          with
          [
              T=nlohmann::json_abi_v3_11_2::detail::from_json_fn
          ]

[facontidavide]

fixed