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`.