update method signature after recent change in up-cpp

[MaximilianToe]

After a recent commit (Commit 7ae7231) to fix several linting errors/warnings in up-cpp the signature of the cleanupListener method of UTransport was changed to use a const reference.
This method is overriden in UTransportDomainSocket; however, now the signature does not match up anymore and will throw an error when compiling.
To make this repository compatible with the latest version of up-cpp, the signature has to also be changed when overriding.

[PLeVasseur]

Thanks for opening the issue!

[PLeVasseur]

Closed via #28 and #30