upas/qer: don't log if mail starts with DKIM-Signature:

Without this, when we use upas/dkim in /mail/lib/qmail:

	upas/dkim ... | upas/qer ...

we'll have a warning in our logs for every message we send.
---
diff f9aa809cbf2d1c17d989bd777c97d4bd4944a8e3 2c6484d1804bd719ae89b9ee36e90b61dd9f6fbb
--- a/sys/src/cmd/upas/q/qer.c
+++ b/sys/src/cmd/upas/q/qer.c
@@ -129,7 +129,7 @@
 	 */
 	i = 0;
 	while((n = read(0, buf, sizeof(buf)-1)) > 0){
-		if(i++ == 0 && strncmp(buf, "From", 4) != 0){
+		if(i++ == 0 && strncmp(buf, "From", 4) != 0 && strncmp(buf, "DKIM-Signature:", 15) != 0){
 			buf[n] = 0;
 			syslog(0, "smtp", "qer usys data starts with %-40.40s", buf);
 		}