Fix -Wshadow warnings in spdlog::sinks::dist_sink

[robinlinden]

This is similar to fbba6df but fixes a few member functions missed in that commit.

spdlog/sinks/dist_sink.h:34:41: error: declaration of 'sink' shadows a member of 'spdlog::sinks::dist_sink<std::mutex>' [-Werror=shadow]
   34 |     void add_sink(std::shared_ptr<sink> sink)
      |                   ~~~~~~~~~~~~~~~~~~~~~~^~~~
In file included from spdlog/sinks/base_sink.h:14,
                 from spdlog/sinks/dist_sink.h:6:
spdlog/sinks/sink.h:13:1: note: shadowed declaration is here
   13 | {
      | ^

[gabime]

Thanks @robinlinden