fix: replace variable 's' with 'source' in deleteSdEventSource

[DespinaRizk]

This PR fixes a variable name typo in Connection::deleteSdEventSource in src/Connection.cpp.

In the #else branch (used when LIBSYSTEMD_VERSION < 243), the code used the variable s instead of the function parameter source:

sd_event_source_set_enabled(s, SD_EVENT_OFF);
sd_event_source_unref(s);

This can lead to a compilation error if s is undefined, or unintended behavior.

The fix replaces s with source to ensure the correct event source is unreferenced.

- sd_event_source_set_enabled(s, SD_EVENT_OFF);
- sd_event_source_unref(s);
+ sd_event_source_set_enabled(source, SD_EVENT_OFF);
+ sd_event_source_unref(source);

[sangelovic]

Thanks @DespinaRizk