Fix userspace build after r302235: do not expose bool field of the
structure, change it to int.

The real fix is to sanitize user-visible definitions in sys/event.h,
e.g. the affected struct knlist is of no use for userspace programs.

Reported and tested by: jkim
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Approved by: re (gjb)