Fix compilation error with GCC 8

[ColinDuquesnoy]

Fix compilation error with GCC 8.0.1

error: need 'typename' before 'std::conditional<std::is_same<char, char>::value, fmt::BasicMemoryWriter<char>, fmt::BasicMemoryWriter<wchar_t> >::type' because 'std::conditional<std::is_same<char, char>::value, fmt::BasicMemoryWriter<char>, fmt::BasicMemoryWriter<wchar_t> >' is a dependent scope
         std::conditional<std::is_same<filename_t::value_type, char>::value, fmt::MemoryWriter, fmt::WMemoryWriter>::type w;