dmesg: inform user --show-delta and iso8601 time format does not mix

References: http://marc.info/?l=util-linux-ng&m=137267606929951&w=2
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

sys-utils/dmesg.c

@@ -1364,6 +1364,9 @@ int main(int argc, char *argv[])
 		case DMESG_TIMEFTM_TIME:
 			ctl.time_fmt = DMESG_TIMEFTM_TIME_DELTA;
 			break;
+		case DMESG_TIMEFTM_ISO8601:
+			warnx(_("--show-delta is ignored when used together with iso8601 time format"));
+			break;
 		default:
 			ctl.time_fmt = DMESG_TIMEFTM_DELTA;
 		}