log: Only use one string buffer

This also fixes a mismatch between date_string and sizeof string in
the strftime call.

src/log.c

@@ -47,18 +47,15 @@ static int weston_log_timestamp(void)
 	gettimeofday(&tv, NULL);
 
 	brokendown_time = localtime(&tv.tv_sec);
-
-	strftime(string, sizeof string, "%H:%M:%S", brokendown_time);
-
 	if (brokendown_time->tm_mday != cached_tm_mday) {
-		char date_string[128];
+		strftime(string, sizeof string, "%Y-%m-%d %Z", brokendown_time);
-
+		fprintf(weston_logfile, "Date: %s\n", string);
-		strftime(date_string, sizeof string, "%Y-%m-%d %Z", brokendown_time);
-		fprintf(weston_logfile, "Date: %s\n", date_string);
 
 		cached_tm_mday = brokendown_time->tm_mday;
 	}
 
+	strftime(string, sizeof string, "%H:%M:%S", brokendown_time);
+
 	return fprintf(weston_logfile, "[%s.%03li] ", string, tv.tv_usec/1000);
 }