MINOR: time: export the global_now variable

MINOR: time: export the global_now variable

This is the process-wide monotonic time that is used to update each
thread's own time. It may be required at a few places where a strictly
monotonic clock is required such as freq_ctr. It will be have to be
backported as a dependency of a forthcoming fix.

(cherry picked from commit 650f374f24c4dd14e90d336c72c2370242be1459)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>