perf stat: Flush output after each line in interval mode

When interval mode is outputting to a pipe, each measurement should be
flushed individually, so that the reader sees it timely.

With a terminal each line is automatically flushed by stdio, but that is
disabled with non terminal output.

Simply fflush output after each time interval

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1375490473-1503-5-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -416,6 +416,8 @@ static void print_interval(void)
 		list_for_each_entry(counter, &evsel_list->entries, node)
 			print_counter_aggr(counter, prefix);
 	}
+
+	fflush(output);
 }
 
 static void handle_initial_delay(void)