[3.0] live555: fix build with versions >= 2024.11.28

In the 2024.11.28 release (version 1732752000), live555 introduced EventLoopWatchVariable as a typedef for std::atomic<char>, replacing direct use of char in earlier versions. Add a conditional typedef to define it as char for older versions, and update event_rtsp and event_data to use EventLoopWatchVariable.

(cherry picked from commit 8befcbfa)

See also: !6972 (merged)