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>