osgEarthCesium: build error with MacOS

[remoe]

Current master doesn't build with cesium-native and Clang:

include/c++/v1/__locale:930:44: error: implicit instantiation of undefined template 'std::ctype<unsigned char>'
    return use_facet<ctype<_CharT> >(__loc).toupper(__c);
                                           ^
osgearth.git/src/osgEarthCesium/CesiumCreditsNode.cpp:50:33: note: in instantiation of function template specialization 'std::toupper<unsigned char>' requested here
                    return std::toupper(c1, locale) == std::toupper(c2, locale);

I've fixed it here: https://github.com/gwaldron/osgearth/pull/2551

[gwaldron]

Fixed in 285eba106