Fix gcc 12 compiling error

[sjanel]

With gcc 12, jwt-cpp does not compile:

[build] include/jwt-cpp/base.h: In static member function ‘static const std::initializer_list<std::__cxx11::basic_string<char> >& jwt::alphabet::helper::base64url_percent_encoding::fill()’:
[build] include/jwt-cpp/base.h:86:100: error: ‘(((const std::__cxx11::basic_string<char>*)(& <temporary>)) != 0)’ is not a constant expression
[build]    86 |                                         static std::initializer_list<std::string> fill{"%3D", "%3d"};

However my other PR fixes the issue much more properly.