fileinfo: ignore USEC if not available
When future porting deprecated code to use g_file_info_get_modification_date_time() we risk a number of breakages because the current implementation also requires the additional use of G_FILE_ATTRIBUTE_TIME_MODIFIED_USEC. This handles that situation gracefully and returns a GDateTime with less precision.
Applications that want the adiditional precision, are already using the additional attribute.