perf stat: Don't write invalid "started on" comment for JSON output

JSON files don't support comments. Disable the "started on" comment when
writing json output to file.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20230408054456.3001367-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -2272,8 +2272,10 @@ int cmd_stat(int argc, const char **argv)
 			perror("failed to create output file");
 			return -1;
 		}
-		clock_gettime(CLOCK_REALTIME, &tm);
-		fprintf(output, "# started on %s\n", ctime(&tm.tv_sec));
+		if (!stat_config.json_output) {
+			clock_gettime(CLOCK_REALTIME, &tm);
+			fprintf(output, "# started on %s\n", ctime(&tm.tv_sec));
+		}
 	} else if (output_fd > 0) {
 		mode = append_file ? "a" : "w";
 		output = fdopen(output_fd, mode);