libevent1: fix layout of duplicated RB_ENTRY() definition

3a509754ded1 removed the color field from our definition, but libevent1
has a copy of it off to the side to prevent event.h consumers from
*needing* to pull in sys/queue.h and sys/tree.h.

Update the event.h definition so that we don't accidentally end up with
two different views of struct event.

This appears to have no functional effect on anything in tree, but this
came up in a local patch to port if_switch(4) and related components
from OpenBSD.

MFC after: 1 week