ofp-print: Omit timestamp when printing packets in ofp_packet_to_string().

There's no value in the timestamp here, because it will always be the
same.  Printing it just makes results less reproducible because output
then depends on the time zone.

This fixes a test failure in California due to yesterday's DST change,
and presumably a test failure almost everywhere else all the time.

Reported-by: Andrew Evans <aevans@nicira.com>

lib/ofp-print.c

@@ -82,7 +82,7 @@ ofp_packet_to_string(const void *data, size_t len, size_t total_len OVS_UNUSED)
     }
     rewind(pcap);
 
-    snprintf(command, sizeof command, "/usr/sbin/tcpdump -e -n -r /dev/fd/%d 2>/dev/null",
+    snprintf(command, sizeof command, "/usr/sbin/tcpdump -t -e -n -r /dev/fd/%d 2>/dev/null",
              fileno(pcap));
     tcpdump = popen(command, "r");
     fclose(pcap);