Inconsistency int/time_t

During the compilation of the Windows Agent (64-bit), the compiler issued a warning about truncation of value. This is quite right, but will happen after Jan. 2038, only. But still, this fix will solve it and there should be no portability problems with my change, because a few lines above the change, the type time_t is used, too. It's a trivial change. Just replaced int by time_t.