BUG/MEDIUM: logs: fix time zone offset format in RFC5424

The time zone offset format used in function update_log_hdr_rfc5424() was
missing ":" as a separator.
diff --git a/src/log.c b/src/log.c
index 6556ab6..3855ae0 100644
--- a/src/log.c
+++ b/src/log.c
@@ -857,9 +857,10 @@
 		get_localtime(tvsec, &tm);
 
 		hdr_len = snprintf(logheader_rfc5424, global.max_syslog_len,
-				   "<<<<>1 %4d-%02d-%02dT%02d:%02d:%02d%s %s ",
+				   "<<<<>1 %4d-%02d-%02dT%02d:%02d:%02d%.3s:%.2s %s ",
 				   tm.tm_year+1900, tm.tm_mon+1, tm.tm_mday,
-				   tm.tm_hour, tm.tm_min, tm.tm_sec, localtimezone,
+				   tm.tm_hour, tm.tm_min, tm.tm_sec,
+				   localtimezone, localtimezone+3,
 				   global.log_send_hostname ? global.log_send_hostname : hostname);
 		/* WARNING: depending upon implementations, snprintf may return
 		 * either -1 or the number of bytes that would be needed to store