MINOR: clock: add clock_get_now_offset() helper

Same as clock_set_now_offset() but to retrieve the offset from external
location.

include/haproxy/clock.h

@@ -50,6 +50,7 @@ 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);
+llong clock_get_now_offset(void);
 
 static inline void clock_update_date(int max_wait, int interrupted)
 {

src/clock.c

@@ -369,6 +369,11 @@ void clock_set_now_offset(llong ofs)
 	HA_ATOMIC_STORE(&now_offset, ofs);
 }
 
+llong clock_get_now_offset(void)
+{
+	return HA_ATOMIC_LOAD(&now_offset);
+}
+
 /* must be called once per thread to initialize their thread-local variables.
  * Note that other threads might also be initializing and running in parallel.
  */