sys/select.h: const'ify the fd_set that __fdset_idx() takes

Some callers may be operating on a const fd_set and we don't
particularly care, so const'ify it.

Reviewed by: kib (earlier version), markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45976