sys: make callout.h self-contained

Avoid including sys/types.h from sys/_callout.h (as we try to avoid having
headers starting with underscores depending on non-underscore headers)
by introducing __sbintime_t in sys/_types.h and including that header
instead.

Reviewed by: imp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33500