Fix TCP_KEEPIDLE undefined on macOS

[fathonix]

TCP_KEEPIDLE is undefined on macOS, this PR fixes the following error when compiling:

../src/Embedded_Display/vncclientthread.cpp:601:43: error: use of undeclared identifier 'TCP_KEEPIDLE'
    if (setsockopt(cl->sock, IPPROTO_TCP, TCP_KEEPIDLE, &optval, optlen) < 0) {
                                          ^

[GermanAizek]

@fathonix many thanks for contributing! I merge your commit in active AQEMU Qt6 fork. I'm going to port AQEMU to Windows, it used to work but stopped. And it is also possible to fix bugs and rewrite to simpler QML code, widgets have too much .ui and C++ code. Weblate integration will be used for translation. https://github.com/AQEMU/aqemu/commit/e68893b0790cc769007fe5dc7b6318ffe19f6f4d I did not touch your authorship commit.