core/timer: downgrade message about random time addition (#5229)

This seems like something that shouldn't be higher then debug level, even
if it does not get emitted too often.

Fixes #5228.

src/core/timer.c

@@ -350,7 +350,7 @@ static void add_random(Timer *t, usec_t *v) {
         else
                 *v += add;
 
-        log_unit_info(UNIT(t), "Adding %s random time.", format_timespan(s, sizeof(s), add, 0));
+        log_unit_debug(UNIT(t), "Adding %s random time.", format_timespan(s, sizeof(s), add, 0));
 }
 
 static void timer_enter_waiting(Timer *t, bool initial) {