get_time_seed(): silence warning emitted by Coverity Scan static analyzer

It warns about the return of time() being truncated to 32 bit, which is
not an issue here.
(this warning was emitted because of the https://github.com/OSGeo/gdal
project embedding a copy of libjson-c and running Coverity Scan
analysis)

random_seed.c

@@ -310,6 +310,7 @@ static int get_time_seed(void)
 {
 	DEBUG_SEED("get_time_seed");
 
+	/* coverity[store_truncates_time_t] */
 	return (unsigned)time(NULL) * 433494437;
 }
 #endif