Fixes compiler errors

[Diskein]

Hi,

I was compiled it from the scratch with gcc (Debian 12.2.0-14) 12.2.0 with -Wall enabled and I got some error:

/opt/src/thirdparty/etcd-cpp-apiv3/src/KeepAlive.cpp:72:52: error: unused parameter 'handler' [-Werror=unused-parameter]
   72 |     std::function<void(std::exception_ptr)> const& handler, int ttl,
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
/opt/src/thirdparty/etcd-cpp-apiv3/src/SyncClient.cpp:150:1: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
  150 | const bool authenticate(std::shared_ptr<grpc::Channel> const& channel,  

The first one is an actual error that can lead to bugs if the ctor will be used. The handler passed to the ctor was omitted by mistake.

[sighingnow]

Merged. Thank you!