build: fix compilation errors when compiling with GCC 11

Errors:

• error: ‘sleep_for’ is not a member of ‘std::this_thread’
• main_eventmon.cpp:109:17: error: ‘int siginterrupt(int, int)’ is deprecated: Use sigaction with SA_RESTART instead [-Werror=deprecated-declarations]

Note: the test test-file-monitor-producer fails if siginterrupt(SIGINT, 1) is removed (see https://gitlab.desy.de/asapo/asapo/-/jobs/453197) and confirms that the changes done in the second commit work to some extent.