MINOR: clock: add clock_set_now_offset() helper

Since now_offset is a static variable and is not exposed outside from
clock.c, let's add an helper so that it becomes possible to set its
value from another source file.

include/haproxy/clock.h

@@ -49,6 +49,7 @@ uint clock_report_idle(void);
 void clock_leaving_poll(int timeout, int interrupted);
 void clock_entering_poll(void);
 void clock_adjust_now_offset(void);
+void clock_set_now_offset(llong ofs);
 
 static inline void clock_update_date(int max_wait, int interrupted)
 {

src/clock.c

@@ -364,6 +364,11 @@ void clock_adjust_now_offset(void)
 	HA_ATOMIC_STORE(&now_offset, now_ns - tv_to_ns(&date));
 }
 
+void clock_set_now_offset(llong ofs)
+{
+	HA_ATOMIC_STORE(&now_offset, ofs);
+}
+
 /* must be called once per thread to initialize their thread-local variables.
  * Note that other threads might also be initializing and running in parallel.
  */