Make `bad_variant_access` inherit publicly from `std::exception`.

[kring]

I'm guessing the (default) private inheritance is just a typo? It causes a C4673 warning in MSVC in our project:

D:\a\cesium-unreal\cesium-unreal\packages\CesiumForUnreal\HostProject\Plugins\CesiumForUnreal\Source\ThirdParty\include\swl\variant.hpp(656): error C4673: throwing 'swl::bad_variant_access' the following types will not be considered at the catch site
D:\a\cesium-unreal\cesium-unreal\packages\CesiumForUnreal\HostProject\Plugins\CesiumForUnreal\Source\CesiumEditor\Private\CesiumIonTokenTroubleshooting.cpp(148): note: see reference to function template instantiation 'Rtype swl::visit<`anonymous-namespace'::isNull::<lambda_1>,const CesiumIonObject&>(Fn &&,const CesiumIonObject &)' being compiled
        with
        [
            Rtype=bool,
            Fn=`anonymous-namespace'::isNull::<lambda_1>
        ]
D:\a\cesium-unreal\cesium-unreal\packages\CesiumForUnreal\HostProject\Plugins\CesiumForUnreal\Source\ThirdParty\include\swl\variant.hpp(656): error C4670: 'exception': this base class is inaccessible

We're currently using swl-variant at the head of main.