MDEV-24115 Fix -Wconversion in Timeval::Timeval() on Mac OS X

The data member tv_usec of the struct timeval is declared as suseconds_t
on MacOS. Size of suseconds_t is 4 bytes. On the other hand, size of ulong
is 8 bytes on 64-bit MacOS, so attempt to assign a value of wider type
(usec) to a value (tv_usec) of narrower type leads to error.

sql/structs.h

@@ -871,7 +871,13 @@ public:
   Timeval(my_time_t sec, ulong usec)
   {
     tv_sec= sec;
-    tv_usec= usec;
+    /*
+      Since tv_usec is not always of type ulong, cast usec parameter
+      explicitly to uint to avoid compiler warnings about losing
+      integer precision.
+    */
+    DBUG_ASSERT(usec < 1000000);
+    tv_usec= (uint)usec;
   }
   explicit Timeval(const timeval &tv)
    :timeval(tv)