allow sol to compile with C++20

[white238]

Fixes a C++20 compilation found in the Smith project on macos.

In file included from smith/axom/src/axom/inlet/LuaReader.cpp:25:
smith/axom/src/thirdparty/axom/sol.hpp:14498:71: error: no template named 'result_of_t' in namespace 'std'
 14498 |                 template <typename R, typename... Args, typename F, typename = std::result_of_t<meta::unqualified_t<F>(Args...)>>
       |                                                                                ~~~~~^
smith/axom/src/thirdparty/axom/sol.hpp:14522:61: error: no template named 'result_of_t' in namespace 'std'
 14522 |                 template <typename... Args, typename F, typename R = std::result_of_t<F&(Args...)>>
       |                                                                      ~~~~~^
smith/axom/src/thirdparty/axom/sol.hpp:20655:86: error: no template named 'result_of_t' in namespace 'std'
 20655 |                 template <typename R, typename... Args, typename Fx, typename Key, typename = std::result_of_t<Fx(Args...)>>
       |                                                                                               ~~~~~^
3 errors generated.

@btalamini

[Arlie-Capps]

Thanks, Chris.