Fix C++20 build resulting in deprecated implicit copy assignment operator warning

[JamesJCode]

Standard has deprecated implicit copy assignment operator creation - just need to specify the default explicitly. Otherwise results in this error in Clang head:

../ExternalLibs/spdlog/include/spdlog/details/log_msg.h:17:5: error: definition of implicit copy assignment operator for 'log_msg' is deprecated because it has a user-declared copy constructor [-Werror,-Wdeprecated-copy]
    log_msg(const log_msg &other) = default;
    ^
../ExternalLibs/spdlog/include/spdlog/details/log_msg_buffer-inl.h:38:14: note: in implicit copy assignment operator for 'spdlog::details::log_msg' first required here
    log_msg::operator=(other);
             ^
1 error generated.

[gabime]

Thanks @JAFJ