fsmonitor: mark some maybe-unused parameters

There's a bit of conditionally-compiled code in fsmonitor, so some function parameters may be unused depending on the build options: - in fsmonitor--daemon.c's try_to_run_foreground_daemon(), we take a detach_console argument, but it's only used on Windows. This seems intentional (and not mistakenly missing other platforms) based on the discussion in c284e27ba7 (fsmonitor--daemon: implement 'start' command, 2022-03-25), which introduced it. - in fsmonitor-setting.c's check_for_incompatible(), we pass the "ipc" flag down to the system-specific fsm_os__incompatible() helper. But we can only do so if our platform has such a helper. In both cases we can mark the argument as MAYBE_UNUSED. That annotates it enough to suppress the compiler's -Wunused-parameter warning, but without making it impossible to use the variable, as a regular UNUSED annotation would. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>