Resolve signed-conversion warnings

If compiled with -Wconversion the compiler complains:
> zmq.hpp:2843:70: warning: implicit conversion changes signedness: 'rep' (aka 'long') to 'size_t' (aka 'unsigned long') [-Wsign-conversion]
>  2843 |         int rc = zmq_timers_set_interval(_timers, timer_id, interval.count());
>       |                  ~~~~~~~~~~~~~~~~~~~~~~~                    ~~~~~~~~~^~~~~~~

Signed-off-by: Soeren Grunewald <soeren.grunewald@desy.de>

Author: soerengrunewald

zmq.hpp

@@ -2899,7 +2899,7 @@ class timers
 
     id_t add(std::chrono::milliseconds interval, zmq_timer_fn handler, void *arg)
     {
-        id_t timer_id = zmq_timers_add(_timers, interval.count(), handler, arg);
+        id_t timer_id = zmq_timers_add(_timers, static_cast<size_t>(interval.count()), handler, arg);
         if (timer_id == -1)
             throw zmq::error_t();
         return timer_id;
@@ -2914,7 +2914,7 @@ class timers
 
     void set_interval(id_t timer_id, std::chrono::milliseconds interval)
     {
-        int rc = zmq_timers_set_interval(_timers, timer_id, interval.count());
+        int rc = zmq_timers_set_interval(_timers, timer_id, static_cast<size_t>(interval.count()));
         if (rc == -1)
             throw zmq::error_t();
     }