tracing: Fix power tracer header

Before patch:
  # tracer: power
  #
  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
  #              | |       |          |         |
  [  676.875865889] CSTATE: Going to C1 on cpu 0 for 0.005911463
  [  676.882938805] CSTATE: Going to C1 on cpu 0 for 0.104796532
  ...

After patch:
  # tracer: power
  #
  #   TIMESTAMP      STATE  EVENT
  #       |            |      |
  [  676.875865889] CSTATE: Going to C1 on cpu 0 for 0.005911463
  [  676.882938805] CSTATE: Going to C1 on cpu 0 for 0.104796532
  ...

v2: Use seq_puts instead of seq_printf
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <49E2E889.5000903@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_power.c

@@ -186,6 +186,12 @@ static enum print_line_t power_print_line(struct trace_iterator *iter)
 	return TRACE_TYPE_UNHANDLED;
 }
 
+static void power_print_header(struct seq_file *s)
+{
+	seq_puts(s, "#   TIMESTAMP      STATE  EVENT\n");
+	seq_puts(s, "#       |            |      |\n");
+}
+
 static struct tracer power_tracer __read_mostly =
 {
 	.name		= "power",
@@ -194,6 +200,7 @@ static struct tracer power_tracer __read_mostly =
 	.stop		= stop_power_trace,
 	.reset		= power_trace_reset,
 	.print_line	= power_print_line,
+	.print_header	= power_print_header,
 };
 
 static int init_power_trace(void)