LinuxKPI: add ktime_get_boottime_seconds()

ktime_get_boottime_seconds() is needed by an updated iwlwifi driver.

Sposored by: The FreeBSD Foundation
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D50005

(cherry picked from commit 93b82146c43b8ecad4fd7694ec1daf6d79cca65c)